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# std.datetime.interval

License:
Boost License 1.0.
Authors:
Jonathan M Davis

Source std/datetime/interval.d

enum `Direction`: int;
  Indicates a direction in time. One example of its use is Interval's expand function which uses it to indicate whether the interval should be expanded backwards (into the past), forwards (into the future), or both.
  `bwd`
    Backward.
  `fwd`
    Forward.
  `both`
    Both backward and forward.
alias `PopFirst` = std.typecons.Flag!"popFirst".Flag;
  Used to indicate whether popFront should be called immediately upon creating a range. The idea is that for some functions used to generate a range for an interval, front is not necessarily a time point which would ever be generated by the range (e.g. if the range were every Sunday within an interval, but the interval started on a Monday), so there needs to be a way to deal with that. To get the first time point in the range to match what the function generates, then use `PopFirst`.yes to indicate that the range should have popFront called on it before the range is returned so that front is a time point which the function would generate. To let the first time point not match the generator function, use PopFront.no.
  For instance, if the function used to generate a range of time points generated successive Easters (i.e. you're iterating over all of the Easters within the interval), the initial date probably isn't an Easter. Using `PopFirst`.yes would tell the function which returned the range that popFront was to be called so that front would then be an Easter - the next one generated by the function (which when iterating forward would be the Easter following the original front, while when iterating backward, it would be the Easter prior to the original front). If `PopFirst`.no were used, then front would remain the original time point and it would not necessarily be a time point which would be generated by the range-generating function (which in many cases is exactly what is desired - e.g. if iterating over every day starting at the beginning of the interval).
  If set to `PopFirst`.no, then popFront is not called before returning the range.
  Otherwise, if set to `PopFirst`.yes, then popFront is called before returning the range.
struct `Interval`(TP);
  Represents an interval of time.
  An `Interval` has a starting point and an end point. The interval of time is therefore the time starting at the starting point up to, but not including, the end point. e.g.

  [January 5th, 2010 - March 10th, 2010)
  [05:00:30 - 12:00:00)
  [1982-01-04T08:59:00 - 2010-07-04T12:00:00)

  A range can be obtained from an `Interval`, allowing iteration over that interval, with the exact time points which are iterated over depending on the function which generates the range.
  pure this(U)(in TP `begin`, in U `end`)
  if (is(Unqual!TP == Unqual!U));
    Parameters:

    TP `begin` | The time point which begins the interval.
    U `end` | The time point which ends (but is not included in) the interval.

    Throws:
    std.datetime.date.DateTimeException if `end` is before `begin`.

    Example

        Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1));

  pure this(D)(in TP `begin`, in D `duration`)
  if (__traits(compiles, `begin` + `duration`));
    Parameters:

    TP `begin` | The time point which begins the interval.
    D `duration` | The `duration` from the starting point to the end point.

    Throws:
    std.datetime.date.DateTimeException if the resulting end is before `begin`.

    Example

        assert(Interval!Date(Date(1996, 1, 2), dur!"days"(3)) ==
               Interval!Date(Date(1996, 1, 2), Date(1996, 1, 5)));

  pure nothrow ref Interval `opAssign`(ref const Interval `rhs`);
    Parameters:

    Interval `rhs` | The Interval to assign to this one.

  pure nothrow ref Interval `opAssign`(Interval `rhs`);
    Parameters:

    Interval `rhs` | The Interval to assign to this one.

  const pure nothrow @property TP `begin`();
    The starting point of the interval. It is included in the interval.

    Example

        assert(Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).begin ==
               Date(1996, 1, 2));

  pure @property void `begin`(TP `timePoint`);
    The starting point of the interval. It is included in the interval.
    Parameters:

    TP `timePoint` | The time point to set `begin` to.

    Throws:
    std.datetime.date.DateTimeException if the resulting interval would be invalid.
  const pure nothrow @property TP `end`();
    The `end` point of the interval. It is excluded from the interval.

    Example

        assert(Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).end ==
               Date(2012, 3, 1));

  pure @property void `end`(TP `timePoint`);
    The `end` point of the interval. It is excluded from the interval.
    Parameters:

    TP `timePoint` | The time point to set `end` to.

    Throws:
    std.datetime.date.DateTimeException if the resulting interval would be invalid.
  const pure nothrow @property auto `length`();
    Returns the duration between begin and end.

    Example

        assert(Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).length ==
               dur!"days"(5903));

  const pure nothrow @property bool `empty`();
    Whether the interval's length is 0, that is, whether begin == end.

    Example

        assert(Interval!Date(Date(1996, 1, 2), Date(1996, 1, 2)).empty);
        assert(!Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).empty);

  const pure bool `contains`(in TP `timePoint`);
    Whether the given time point is within this interval.
    Parameters:

    TP `timePoint` | The time point to check for inclusion in this interval.

    Throws:
    std.datetime.date.DateTimeException if this interval is empty.

    Example

        assert(!Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).contains(
                    Date(1994, 12, 24)));

        assert(Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).contains(
                    Date(2000, 1, 5)));
        assert(!Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).contains(
                    Date(2012, 3, 1)));

  const pure bool `contains`(in Interval `interval`);
    Whether the given `interval` is completely within this `interval`.
    Parameters:

    Interval `interval` | The `interval` to check for inclusion in this `interval`.

    Throws:
    std.datetime.date.DateTimeException if either `interval` is empty.

    Example

        assert(!Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).contains(
                    Interval!Date(Date(1990, 7, 6), Date(2000, 8, 2))));

        assert(Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).contains(
                    Interval!Date(Date(1999, 1, 12), Date(2011, 9, 17))));

        assert(!Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).contains(
                    Interval!Date(Date(1998, 2, 28), Date(2013, 5, 1))));

  const pure bool `contains`(in PosInfInterval!TP `interval`);
    Whether the given `interval` is completely within this `interval`.
    Always returns `false` (unless this `interval` is empty), because an `interval` going to positive infinity can never be contained in a finite `interval`.
    Parameters:

    PosInfInterval!TP `interval` | The `interval` to check for inclusion in this `interval`.

    Throws:
    std.datetime.date.DateTimeException if this `interval` is empty.

    Example

        assert(!Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).contains(
                    PosInfInterval!Date(Date(1999, 5, 4))));

  const pure bool `contains`(in NegInfInterval!TP `interval`);
    Whether the given `interval` is completely within this `interval`.
    Always returns `false` (unless this `interval` is empty), because an `interval` beginning at negative infinity can never be contained in a finite `interval`.
    Parameters:

    NegInfInterval!TP `interval` | The `interval` to check for inclusion in this `interval`.

    Throws:
    std.datetime.date.DateTimeException if this `interval` is empty.

    Example

        assert(!Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).contains(
                    NegInfInterval!Date(Date(1996, 5, 4))));

  const pure bool `isBefore`(in TP `timePoint`);
    Whether this interval is before the given time point.
    Parameters:

    TP `timePoint` | The time point to check whether this interval is before it.

    Throws:
    std.datetime.date.DateTimeException if this interval is empty.

    Example

        assert(!Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).isBefore(
                    Date(1994, 12, 24)));

        assert(!Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).isBefore(
                    Date(2000, 1, 5)));

        assert(Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).isBefore(
                    Date(2012, 3, 1)));

  const pure bool `isBefore`(in Interval `interval`);
    Whether this `interval` is before the given `interval` and does not intersect with it.
    Parameters:

    Interval `interval` | The `interval` to check for against this `interval`.

    Throws:
    std.datetime.date.DateTimeException if either `interval` is empty.

    Example

        assert(!Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).isBefore(
                    Interval!Date(Date(1990, 7, 6), Date(2000, 8, 2))));

        assert(!Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).isBefore(
                    Interval!Date(Date(1999, 1, 12), Date(2011, 9, 17))));

        assert(Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).isBefore(
                    Interval!Date(Date(2012, 3, 1), Date(2013, 5, 1))));

  const pure bool `isBefore`(in PosInfInterval!TP `interval`);
    Whether this `interval` is before the given `interval` and does not intersect with it.
    Parameters:

    PosInfInterval!TP `interval` | The `interval` to check for against this `interval`.

    Throws:
    std.datetime.date.DateTimeException if this `interval` is empty.

    Example

        assert(!Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).isBefore(
                    PosInfInterval!Date(Date(1999, 5, 4))));

        assert(Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).isBefore(
                    PosInfInterval!Date(Date(2013, 3, 7))));

  const pure bool `isBefore`(in NegInfInterval!TP `interval`);
    Whether this `interval` is before the given `interval` and does not intersect with it.
    Always returns `false` (unless this `interval` is empty) because a finite `interval` can never be before an `interval` beginning at negative infinity.
    Parameters:

    NegInfInterval!TP `interval` | The `interval` to check for against this `interval`.

    Throws:
    std.datetime.date.DateTimeException if this `interval` is empty.

    Example

        assert(!Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).isBefore(
                    NegInfInterval!Date(Date(1996, 5, 4))));

  const pure bool `isAfter`(in TP `timePoint`);
    Whether this interval is after the given time point.
    Parameters:

    TP `timePoint` | The time point to check whether this interval is after it.

    Throws:
    std.datetime.date.DateTimeException if this interval is empty.

    Example

        assert(Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).isAfter(
                    Date(1994, 12, 24)));

        assert(!Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).isAfter(
                    Date(2000, 1, 5)));

        assert(!Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).isAfter(
                    Date(2012, 3, 1)));

  const pure bool `isAfter`(in Interval `interval`);
    Whether this `interval` is after the given `interval` and does not intersect it.
    Parameters:

    Interval `interval` | The `interval` to check against this `interval`.

    Throws:
    std.datetime.date.DateTimeException if either `interval` is empty.

    Example

        assert(!Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).isAfter(
                    Interval!Date(Date(1990, 7, 6), Date(2000, 8, 2))));

        assert(!Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).isAfter(
                    Interval!Date(Date(1999, 1, 12), Date(2011, 9, 17))));

        assert(Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).isAfter(
                    Interval!Date(Date(1989, 3, 1), Date(1996, 1, 2))));

  const pure bool `isAfter`(in PosInfInterval!TP `interval`);
    Whether this `interval` is after the given `interval` and does not intersect it.
    Always returns `false` (unless this `interval` is empty) because a finite `interval` can never be after an `interval` going to positive infinity.
    Parameters:

    PosInfInterval!TP `interval` | The `interval` to check against this `interval`.

    Throws:
    std.datetime.date.DateTimeException if this `interval` is empty.

    Example

        assert(!Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).isAfter(
                    PosInfInterval!Date(Date(1999, 5, 4))));

  const pure bool `isAfter`(in NegInfInterval!TP `interval`);
    Whether this `interval` is after the given `interval` and does not intersect it.
    Parameters:

    NegInfInterval!TP `interval` | The `interval` to check against this `interval`.

    Throws:
    std.datetime.date.DateTimeException if this `interval` is empty.

    Example

        assert(Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).isAfter(
                    NegInfInterval!Date(Date(1996, 1, 2))));

  const pure bool `intersects`(in Interval `interval`);
    Whether the given `interval` overlaps this `interval`.
    Parameters:

    Interval `interval` | The `interval` to check for intersection with this `interval`.

    Throws:
    std.datetime.date.DateTimeException if either `interval` is empty.

    Example

        assert(Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).intersects(
                    Interval!Date(Date(1990, 7, 6), Date(2000, 8, 2))));

        assert(Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).intersects(
                    Interval!Date(Date(1999, 1, 12), Date(2011, 9, 17))));

        assert(!Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).intersects(
                    Interval!Date(Date(1989, 3, 1), Date(1996, 1, 2))));

  const pure bool `intersects`(in PosInfInterval!TP `interval`);
    Whether the given `interval` overlaps this `interval`.
    Parameters:

    PosInfInterval!TP `interval` | The `interval` to check for intersection with this `interval`.

    Throws:
    std.datetime.date.DateTimeException if this `interval` is empty.

    Example

        assert(Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).intersects(
                    PosInfInterval!Date(Date(1999, 5, 4))));

        assert(!Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).intersects(
                    PosInfInterval!Date(Date(2012, 3, 1))));

  const pure bool `intersects`(in NegInfInterval!TP `interval`);
    Whether the given `interval` overlaps this `interval`.
    Parameters:

    NegInfInterval!TP `interval` | The `interval` to check for intersection with this `interval`.

    Throws:
    std.datetime.date.DateTimeException if this `interval` is empty.

    Example

        assert(!Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).intersects(
                    NegInfInterval!Date(Date(1996, 1, 2))));

        assert(Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).intersects(
                    NegInfInterval!Date(Date(2000, 1, 2))));

  const Interval `intersection`(in Interval `interval`);
    Returns the `intersection` of two intervals
    Parameters:

    Interval `interval` | The `interval` to intersect with this `interval`.

    Throws:
    std.datetime.date.DateTimeException if the two intervals do not intersect or if either `interval` is empty.

    Example

        assert(Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).intersection(
                    Interval!Date(Date(1990, 7, 6), Date(2000, 8, 2))) ==
               Interval!Date(Date(1996, 1 , 2), Date(2000, 8, 2)));

        assert(Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).intersection(
                    Interval!Date(Date(1999, 1, 12), Date(2011, 9, 17))) ==
               Interval!Date(Date(1999, 1 , 12), Date(2011, 9, 17)));

  const Interval `intersection`(in PosInfInterval!TP `interval`);
    Returns the `intersection` of two intervals
    Parameters:

    PosInfInterval!TP `interval` | The `interval` to intersect with this `interval`.

    Throws:
    std.datetime.date.DateTimeException if the two intervals do not intersect or if this `interval` is empty.

    Example

        assert(Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).intersection(
                    PosInfInterval!Date(Date(1990, 7, 6))) ==
               Interval!Date(Date(1996, 1 , 2), Date(2012, 3, 1)));

        assert(Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).intersection(
                    PosInfInterval!Date(Date(1999, 1, 12))) ==
               Interval!Date(Date(1999, 1 , 12), Date(2012, 3, 1)));

  const Interval `intersection`(in NegInfInterval!TP `interval`);
    Returns the `intersection` of two intervals
    Parameters:

    NegInfInterval!TP `interval` | The `interval` to intersect with this `interval`.

    Throws:
    std.datetime.date.DateTimeException if the two intervals do not intersect or if this `interval` is empty.

    Example

        assert(Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).intersection(
                    NegInfInterval!Date(Date(1999, 7, 6))) ==
               Interval!Date(Date(1996, 1 , 2), Date(1999, 7, 6)));

        assert(Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).intersection(
                    NegInfInterval!Date(Date(2013, 1, 12))) ==
               Interval!Date(Date(1996, 1 , 2), Date(2012, 3, 1)));

  const pure bool `isAdjacent`(in Interval `interval`);
    Whether the given `interval` is adjacent to this `interval`.
    Parameters:

    Interval `interval` | The `interval` to check whether its adjecent to this `interval`.

    Throws:
    std.datetime.date.DateTimeException if either `interval` is empty.

    Example

        assert(Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).isAdjacent(
                    Interval!Date(Date(1990, 7, 6), Date(1996, 1, 2))));

        assert(Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).isAdjacent(
                    Interval!Date(Date(2012, 3, 1), Date(2013, 9, 17))));

        assert(!Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).isAdjacent(
                    Interval!Date(Date(1989, 3, 1), Date(2012, 3, 1))));

  const pure bool `isAdjacent`(in PosInfInterval!TP `interval`);
    Whether the given `interval` is adjacent to this `interval`.
    Parameters:

    PosInfInterval!TP `interval` | The `interval` to check whether its adjecent to this `interval`.

    Throws:
    std.datetime.date.DateTimeException if this `interval` is empty.

    Example

        assert(!Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).isAdjacent(
                    PosInfInterval!Date(Date(1999, 5, 4))));

        assert(Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).isAdjacent(
                    PosInfInterval!Date(Date(2012, 3, 1))));

  const pure bool `isAdjacent`(in NegInfInterval!TP `interval`);
    Whether the given `interval` is adjacent to this `interval`.
    Parameters:

    NegInfInterval!TP `interval` | The `interval` to check whether its adjecent to this `interval`.

    Throws:
    std.datetime.date.DateTimeException if this `interval` is empty.

    Example

        assert(Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).isAdjacent(
                    NegInfInterval!Date(Date(1996, 1, 2))));

        assert(!Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).isAdjacent(
                    NegInfInterval!Date(Date(2000, 1, 2))));

  const Interval `merge`(in Interval `interval`);
    Returns the union of two intervals
    Parameters:

    Interval `interval` | The `interval` to `merge` with this `interval`.

    Throws:
    std.datetime.date.DateTimeException if the two intervals do not intersect and are not adjacent or if either `interval` is empty.

    Example

        assert(Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).merge(
                    Interval!Date(Date(1990, 7, 6), Date(2000, 8, 2))) ==
               Interval!Date(Date(1990, 7 , 6), Date(2012, 3, 1)));

        assert(Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).merge(
                    Interval!Date(Date(2012, 3, 1), Date(2013, 5, 7))) ==
               Interval!Date(Date(1996, 1 , 2), Date(2013, 5, 7)));

  const PosInfInterval!TP `merge`(in PosInfInterval!TP `interval`);
    Returns the union of two intervals
    Parameters:

    PosInfInterval!TP `interval` | The `interval` to `merge` with this `interval`.

    Throws:
    std.datetime.date.DateTimeException if the two intervals do not intersect and are not adjacent or if this `interval` is empty.

    Example

        assert(Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).merge(
                    PosInfInterval!Date(Date(1990, 7, 6))) ==
               PosInfInterval!Date(Date(1990, 7 , 6)));

        assert(Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).merge(
                    PosInfInterval!Date(Date(2012, 3, 1))) ==
               PosInfInterval!Date(Date(1996, 1 , 2)));

  const NegInfInterval!TP `merge`(in NegInfInterval!TP `interval`);
    Returns the union of two intervals
    Parameters:

    NegInfInterval!TP `interval` | The `interval` to `merge` with this `interval`.

    Throws:
    std.datetime.date.DateTimeException if the two intervals do not intersect and are not adjacent or if this `interval` is empty.

    Example

        assert(Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).merge(
                    NegInfInterval!Date(Date(1996, 1, 2))) ==
               NegInfInterval!Date(Date(2012, 3 , 1)));

        assert(Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).merge(
                    NegInfInterval!Date(Date(2013, 1, 12))) ==
               NegInfInterval!Date(Date(2013, 1 , 12)));

  const pure Interval `span`(in Interval `interval`);
    Returns an `interval` that covers from the earliest time point of two intervals up to (but not including) the latest time point of two intervals.
    Parameters:

    Interval `interval` | The `interval` to create a `span` together with this `interval`.

    Throws:
    std.datetime.date.DateTimeException if either `interval` is empty.

    Example

        assert(Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).span(
                    Interval!Date(Date(1990, 7, 6), Date(1991, 1, 8))) ==
               Interval!Date(Date(1990, 7 , 6), Date(2012, 3, 1)));

        assert(Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).span(
                    Interval!Date(Date(2012, 3, 1), Date(2013, 5, 7))) ==
               Interval!Date(Date(1996, 1 , 2), Date(2013, 5, 7)));

  const pure PosInfInterval!TP `span`(in PosInfInterval!TP `interval`);
    Returns an `interval` that covers from the earliest time point of two intervals up to (but not including) the latest time point of two intervals.
    Parameters:

    PosInfInterval!TP `interval` | The `interval` to create a `span` together with this `interval`.

    Throws:
    std.datetime.date.DateTimeException if this `interval` is empty.

    Example

        assert(Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).span(
                    PosInfInterval!Date(Date(1990, 7, 6))) ==
               PosInfInterval!Date(Date(1990, 7 , 6)));

        assert(Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).span(
                    PosInfInterval!Date(Date(2050, 1, 1))) ==
               PosInfInterval!Date(Date(1996, 1 , 2)));

  const pure NegInfInterval!TP `span`(in NegInfInterval!TP `interval`);
    Returns an `interval` that covers from the earliest time point of two intervals up to (but not including) the latest time point of two intervals.
    Parameters:

    NegInfInterval!TP `interval` | The `interval` to create a `span` together with this `interval`.

    Throws:
    std.datetime.date.DateTimeException if this `interval` is empty.

    Example

        assert(Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).span(
                    NegInfInterval!Date(Date(1602, 5, 21))) ==
               NegInfInterval!Date(Date(2012, 3 , 1)));

        assert(Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).span(
                    NegInfInterval!Date(Date(2013, 1, 12))) ==
               NegInfInterval!Date(Date(2013, 1 , 12)));

  pure void `shift`(D)(D `duration`)
  if (__traits(compiles, begin + `duration`));
    Shifts the interval forward or backwards in time by the given `duration` (a positive `duration` shifts the interval forward; a negative `duration` shifts it backward). Effectively, it does begin += `duration` and end += `duration`.
    Parameters:

    D `duration` | The `duration` to `shift` the interval by.

    Throws:
    std.datetime.date.DateTimeException this interval is empty or if the resulting interval would be invalid.

    Example

        auto interval1 = Interval!Date(Date(1996, 1, 2), Date(2012, 4, 5));
        auto interval2 = Interval!Date(Date(1996, 1, 2), Date(2012, 4, 5));

        interval1.shift(dur!"days"(50));
        assert(interval1 == Interval!Date(Date(1996, 2, 21), Date(2012, 5, 25)));

        interval2.shift(dur!"days"(-50));
        assert(interval2 == Interval!Date(Date(1995, 11, 13), Date(2012, 2, 15)));

  void `shift`(T)(T `years`, T `months` = 0, AllowDayOverflow `allowOverflow` = AllowDayOverflow.yes)
  if (isIntegral!T);
    Shifts the interval forward or backwards in time by the given number of `years` and/or `months` (a positive number of `years` and `months` shifts the interval forward; a negative number shifts it backward). It adds the `years` the given `years` and `months` to both begin and end. It effectively calls add!"`years`"() and then add!"`months`"() on begin and end with the given number of `years` and `months`.
    Parameters:

    T `years` | The number of `years` to `shift` the interval by.
    T `months` | The number of `months` to `shift` the interval by.
    AllowDayOverflow `allowOverflow` | Whether the days should be allowed to overflow on begin and end, causing their month to increment.

    Throws:
    std.datetime.date.DateTimeException if this interval is empty or if the resulting interval would be invalid.

    Example

        auto interval1 = Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1));
        auto interval2 = Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1));

        interval1.shift(2);
        assert(interval1 == Interval!Date(Date(1998, 1, 2), Date(2014, 3, 1)));

        interval2.shift(-2);
        assert(interval2 == Interval!Date(Date(1994, 1, 2), Date(2010, 3, 1)));

  pure void `expand`(D)(D `duration`, Direction `dir` = Direction.both)
  if (__traits(compiles, begin + `duration`));
    Expands the interval forwards and/or backwards in time. Effectively, it does begin -= `duration` and/or end += `duration`. Whether it expands forwards and/or backwards in time is determined by `dir`.
    Parameters:

    D `duration` | The `duration` to `expand` the interval by.
    Direction `dir` | The direction in time to `expand` the interval.

    Throws:
    std.datetime.date.DateTimeException this interval is empty or if the resulting interval would be invalid.

    Example

        auto interval1 = Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1));
        auto interval2 = Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1));

        interval1.expand(2);
        assert(interval1 == Interval!Date(Date(1994, 1, 2), Date(2014, 3, 1)));

        interval2.expand(-2);
        assert(interval2 == Interval!Date(Date(1998, 1, 2), Date(2010, 3, 1)));

  void `expand`(T)(T `years`, T `months` = 0, AllowDayOverflow `allowOverflow` = AllowDayOverflow.yes, Direction `dir` = Direction.both)
  if (isIntegral!T);
    Expands the interval forwards and/or backwards in time. Effectively, it subtracts the given number of `months`/`years` from begin and adds them to end. Whether it expands forwards and/or backwards in time is determined by `dir`.
    Parameters:

    T `years` | The number of `years` to `expand` the interval by.
    T `months` | The number of `months` to `expand` the interval by.
    AllowDayOverflow `allowOverflow` | Whether the days should be allowed to overflow on begin and end, causing their month to increment.
    Direction `dir` | The direction in time to `expand` the interval.

    Throws:
    std.datetime.date.DateTimeException if this interval is empty or if the resulting interval would be invalid.

    Example

        auto interval1 = Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1));
        auto interval2 = Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1));

        interval1.expand(2);
        assert(interval1 == Interval!Date(Date(1994, 1, 2), Date(2014, 3, 1)));

        interval2.expand(-2);
        assert(interval2 == Interval!Date(Date(1998, 1, 2), Date(2010, 3, 1)));

  const IntervalRange!(TP, Direction.fwd) `fwdRange`(TP delegate(in TP) `func`, PopFirst `popFirst` = PopFirst.no);
    Returns a range which iterates forward over the interval, starting at begin, using `func` to generate each successive time point.
    The range's front is the interval's begin. `func` is used to generate the next front when popFront is called. If `popFirst` is PopFirst.yes, then popFront is called before the range is returned (so that front is a time point which `func` would generate).
    If `func` ever generates a time point less than or equal to the current front of the range, then a std.datetime.date.DateTimeException will be thrown. The range will be empty and iteration complete when `func` generates a time point equal to or beyond the end of the interval.
    There are helper functions in this module which generate common delegates to pass to `fwdRange`. Their documentation starts with "Range-generating function," making them easily searchable.
    Parameters:

    TP delegate(in TP) `func` | The function used to generate the time points of the range over the interval.
    PopFirst `popFirst` | Whether popFront should be called on the range before returning it.

    Throws:
    std.datetime.date.DateTimeException if this interval is empty.

    Warning `func` must be logically pure. Ideally, `func` would be a function pointer to a pure function, but forcing `func` to be pure is far too restrictive to be useful, and in order to have the ease of use of having functions which generate functions to pass to `fwdRange`, `func` must be a delegate.

    If `func` retains state which changes as it is called, then some algorithms will not work correctly, because the range's save will have failed to have really saved the range's state. To avoid such bugs, don't pass a delegate which is not logically pure to `fwdRange`. If `func` is given the same time point with two different calls, it must return the same result both times.
    Of course, none of the functions in this module have this problem, so it's only relevant if when creating a custom delegate.

    Example

        auto interval = Interval!Date(Date(2010, 9, 1), Date(2010, 9, 9));
        auto func = delegate (in Date date) // For iterating over even-numbered days.
                    {
                        if ((date.day & 1) == 0)
                            return date + dur!"days"(2);

                        return date + dur!"days"(1);
                    };
        auto range = interval.fwdRange(func);

        // An odd day. Using PopFirst.yes would have made this Date(2010, 9, 2).
        assert(range.front == Date(2010, 9, 1));

        range.popFront();
        assert(range.front == Date(2010, 9, 2));

        range.popFront();
        assert(range.front == Date(2010, 9, 4));

        range.popFront();
        assert(range.front == Date(2010, 9, 6));

        range.popFront();
        assert(range.front == Date(2010, 9, 8));

        range.popFront();
        assert(range.empty);

  const IntervalRange!(TP, Direction.bwd) `bwdRange`(TP delegate(in TP) `func`, PopFirst `popFirst` = PopFirst.no);
    Returns a range which iterates backwards over the interval, starting at end, using `func` to generate each successive time point.
    The range's front is the interval's end. `func` is used to generate the next front when popFront is called. If `popFirst` is PopFirst.yes, then popFront is called before the range is returned (so that front is a time point which `func` would generate).
    If `func` ever generates a time point greater than or equal to the current front of the range, then a std.datetime.date.DateTimeException will be thrown. The range will be empty and iteration complete when `func` generates a time point equal to or less than the begin of the interval.
    There are helper functions in this module which generate common delegates to pass to `bwdRange`. Their documentation starts with "Range-generating function," making them easily searchable.
    Parameters:

    TP delegate(in TP) `func` | The function used to generate the time points of the range over the interval.
    PopFirst `popFirst` | Whether popFront should be called on the range before returning it.

    Throws:
    std.datetime.date.DateTimeException if this interval is empty.

    Warning `func` must be logically pure. Ideally, `func` would be a function pointer to a pure function, but forcing `func` to be pure is far too restrictive to be useful, and in order to have the ease of use of having functions which generate functions to pass to fwdRange, `func` must be a delegate.

    If `func` retains state which changes as it is called, then some algorithms will not work correctly, because the range's save will have failed to have really saved the range's state. To avoid such bugs, don't pass a delegate which is not logically pure to fwdRange. If `func` is given the same time point with two different calls, it must return the same result both times.
    Of course, none of the functions in this module have this problem, so it's only relevant for custom delegates.

    Example

        auto interval = Interval!Date(Date(2010, 9, 1), Date(2010, 9, 9));
        auto func = delegate (in Date date) // For iterating over even-numbered days.
                    {
                        if ((date.day & 1) == 0)
                            return date - dur!"days"(2);

                        return date - dur!"days"(1);
                    };
        auto range = interval.bwdRange(func);

        // An odd day. Using PopFirst.yes would have made this Date(2010, 9, 8).
        assert(range.front == Date(2010, 9, 9));

        range.popFront();
        assert(range.front == Date(2010, 9, 8));

        range.popFront();
        assert(range.front == Date(2010, 9, 6));

        range.popFront();
        assert(range.front == Date(2010, 9, 4));

        range.popFront();
        assert(range.front == Date(2010, 9, 2));

        range.popFront();
        assert(range.empty);

  const nothrow string `toString`();
    Converts this interval to a string.
struct `PosInfInterval`(TP);
  Represents an interval of time which has positive infinity as its end point.
  Any ranges which iterate over a `PosInfInterval` are infinite. So, the main purpose of using `PosInfInterval` is to create an infinite range which starts at a fixed point in time and goes to positive infinity.
  pure nothrow this(in TP `begin`);
    Parameters:

    TP `begin` | The time point which begins the interval.

    Example

        auto interval = PosInfInterval!Date(Date(1996, 1, 2));

  pure nothrow ref PosInfInterval `opAssign`(ref const PosInfInterval `rhs`);
    Parameters:

    PosInfInterval `rhs` | The PosInfInterval to assign to this one.

  pure nothrow ref PosInfInterval `opAssign`(PosInfInterval `rhs`);
    Parameters:

    PosInfInterval `rhs` | The PosInfInterval to assign to this one.

  const pure nothrow @property TP `begin`();
    The starting point of the interval. It is included in the interval.

    Example

        assert(PosInfInterval!Date(Date(1996, 1, 2)).begin == Date(1996, 1, 2));

  pure nothrow @property void `begin`(TP `timePoint`);
    The starting point of the interval. It is included in the interval.
    Parameters:

    TP `timePoint` | The time point to set `begin` to.

  enum bool `empty`;
    Whether the interval's length is 0. Always returns `false`.

    Example

        assert(!PosInfInterval!Date(Date(1996, 1, 2)).empty);

  const pure nothrow bool `contains`(TP `timePoint`);
    Whether the given time point is within this interval.
    Parameters:

    TP `timePoint` | The time point to check for inclusion in this interval.

    Example

        assert(!PosInfInterval!Date(Date(1996, 1, 2)).contains(Date(1994, 12, 24)));
        assert(PosInfInterval!Date(Date(1996, 1, 2)).contains(Date(2000, 1, 5)));

  const pure bool `contains`(in Interval!TP `interval`);
    Whether the given `interval` is completely within this `interval`.
    Parameters:

    Interval!TP `interval` | The `interval` to check for inclusion in this `interval`.

    Throws:
    std.datetime.date.DateTimeException if the given `interval` is empty.

    Example

        assert(!PosInfInterval!Date(Date(1996, 1, 2)).contains(
                    Interval!Date(Date(1990, 7, 6), Date(2000, 8, 2))));

        assert(PosInfInterval!Date(Date(1996, 1, 2)).contains(
                    Interval!Date(Date(1999, 1, 12), Date(2011, 9, 17))));

        assert(PosInfInterval!Date(Date(1996, 1, 2)).contains(
                    Interval!Date(Date(1998, 2, 28), Date(2013, 5, 1))));

  const pure nothrow bool `contains`(in PosInfInterval `interval`);
    Whether the given `interval` is completely within this `interval`.
    Parameters:

    PosInfInterval `interval` | The `interval` to check for inclusion in this `interval`.

    Example

        assert(PosInfInterval!Date(Date(1996, 1, 2)).contains(
                    PosInfInterval!Date(Date(1999, 5, 4))));

        assert(!PosInfInterval!Date(Date(1996, 1, 2)).contains(
                    PosInfInterval!Date(Date(1995, 7, 2))));

  const pure nothrow bool `contains`(in NegInfInterval!TP `interval`);
    Whether the given `interval` is completely within this `interval`.
    Always returns `false` because an `interval` going to positive infinity can never contain an `interval` beginning at negative infinity.
    Parameters:

    NegInfInterval!TP `interval` | The `interval` to check for inclusion in this `interval`.

    Example

        assert(!PosInfInterval!Date(Date(1996, 1, 2)).contains(
                    NegInfInterval!Date(Date(1996, 5, 4))));

  const pure nothrow bool `isBefore`(in TP `timePoint`);
    Whether this interval is before the given time point.
    Always returns `false` because an interval going to positive infinity can never be before any time point.
    Parameters:

    TP `timePoint` | The time point to check whether this interval is before it.

    Example

        assert(!PosInfInterval!Date(Date(1996, 1, 2)).isBefore(Date(1994, 12, 24)));
        assert(!PosInfInterval!Date(Date(1996, 1, 2)).isBefore(Date(2000, 1, 5)));

  const pure bool `isBefore`(in Interval!TP `interval`);
    Whether this `interval` is before the given `interval` and does not intersect it.
    Always returns `false` (unless the given `interval` is empty) because an `interval` going to positive infinity can never be before any other `interval`.
    Parameters:

    Interval!TP `interval` | The `interval` to check for against this `interval`.

    Throws:
    std.datetime.date.DateTimeException if the given `interval` is empty.

    Example

        assert(!PosInfInterval!Date(Date(1996, 1, 2)).isBefore(
                    Interval!Date(Date(1990, 7, 6), Date(2000, 8, 2))));

        assert(!PosInfInterval!Date(Date(1996, 1, 2)).isBefore(
                    Interval!Date(Date(1999, 1, 12), Date(2011, 9, 17))));

  const pure nothrow bool `isBefore`(in PosInfInterval `interval`);
    Whether this `interval` is before the given `interval` and does not intersect it.
    Always returns `false` because an `interval` going to positive infinity can never be before any other `interval`.
    Parameters:

    PosInfInterval `interval` | The `interval` to check for against this `interval`.

    Example

        assert(!PosInfInterval!Date(Date(1996, 1, 2)).isBefore(
                    PosInfInterval!Date(Date(1992, 5, 4))));

        assert(!PosInfInterval!Date(Date(1996, 1, 2)).isBefore(
                    PosInfInterval!Date(Date(2013, 3, 7))));

  const pure nothrow bool `isBefore`(in NegInfInterval!TP `interval`);
    Whether this `interval` is before the given `interval` and does not intersect it.
    Always returns `false` because an `interval` going to positive infinity can never be before any other `interval`.
    Parameters:

    NegInfInterval!TP `interval` | The `interval` to check for against this `interval`.

    Example

        assert(!PosInfInterval!Date(Date(1996, 1, 2)).isBefore(
                    NegInfInterval!Date(Date(1996, 5, 4))));

  const pure nothrow bool `isAfter`(in TP `timePoint`);
    Whether this interval is after the given time point.
    Parameters:

    TP `timePoint` | The time point to check whether this interval is after it.

    Example

        assert(PosInfInterval!Date(Date(1996, 1, 2)).isAfter(Date(1994, 12, 24)));
        assert(!PosInfInterval!Date(Date(1996, 1, 2)).isAfter(Date(2000, 1, 5)));

  const pure bool `isAfter`(in Interval!TP `interval`);
    Whether this `interval` is after the given `interval` and does not intersect it.
    Parameters:

    Interval!TP `interval` | The `interval` to check against this `interval`.

    Throws:
    std.datetime.date.DateTimeException if the given `interval` is empty.

    Example

        assert(!PosInfInterval!Date(Date(1996, 1, 2)).isAfter(
                    Interval!Date(Date(1990, 7, 6), Date(2000, 8, 2))));

        assert(!PosInfInterval!Date(Date(1996, 1, 2)).isAfter(
                    Interval!Date(Date(1999, 1, 12), Date(2011, 9, 17))));

        assert(PosInfInterval!Date(Date(1996, 1, 2)).isAfter(
                    Interval!Date(Date(1989, 3, 1), Date(1996, 1, 2))));

  const pure nothrow bool `isAfter`(in PosInfInterval `interval`);
    Whether this `interval` is after the given `interval` and does not intersect it.
    Always returns `false` because an `interval` going to positive infinity can never be after another `interval` going to positive infinity.
    Parameters:

    PosInfInterval `interval` | The `interval` to check against this `interval`.

    Example

        assert(!PosInfInterval!Date(Date(1996, 1, 2)).isAfter(
                    PosInfInterval!Date(Date(1990, 1, 7))));

        assert(!PosInfInterval!Date(Date(1996, 1, 2)).isAfter(
                    PosInfInterval!Date(Date(1999, 5, 4))));

  const pure nothrow bool `isAfter`(in NegInfInterval!TP `interval`);
    Whether this `interval` is after the given `interval` and does not intersect it.
    Parameters:

    NegInfInterval!TP `interval` | The `interval` to check against this `interval`.

    Example

        assert(PosInfInterval!Date(Date(1996, 1, 2)).isAfter(
                    NegInfInterval!Date(Date(1996, 1, 2))));

        assert(!PosInfInterval!Date(Date(1996, 1, 2)).isAfter(
                    NegInfInterval!Date(Date(2000, 7, 1))));

  const pure bool `intersects`(in Interval!TP `interval`);
    Whether the given `interval` overlaps this `interval`.
    Parameters:

    Interval!TP `interval` | The `interval` to check for intersection with this `interval`.

    Throws:
    std.datetime.date.DateTimeException if the given `interval` is empty.

    Example

        assert(PosInfInterval!Date(Date(1996, 1, 2)).intersects(
                    Interval!Date(Date(1990, 7, 6), Date(2000, 8, 2))));

        assert(PosInfInterval!Date(Date(1996, 1, 2)).intersects(
                    Interval!Date(Date(1999, 1, 12), Date(2011, 9, 17))));

        assert(!PosInfInterval!Date(Date(1996, 1, 2)).intersects(
                    Interval!Date(Date(1989, 3, 1), Date(1996, 1, 2))));

  const pure nothrow bool `intersects`(in PosInfInterval `interval`);
    Whether the given `interval` overlaps this `interval`.
    Always returns `true` because two intervals going to positive infinity always overlap.
    Parameters:

    PosInfInterval `interval` | The `interval` to check for intersection with this `interval`.

    Example

        assert(PosInfInterval!Date(Date(1996, 1, 2)).intersects(
                    PosInfInterval!Date(Date(1990, 1, 7))));

        assert(PosInfInterval!Date(Date(1996, 1, 2)).intersects(
                    PosInfInterval!Date(Date(1999, 5, 4))));

  const pure nothrow bool `intersects`(in NegInfInterval!TP `interval`);
    Whether the given `interval` overlaps this `interval`.
    Parameters:

    NegInfInterval!TP `interval` | The `interval` to check for intersection with this `interval`.

    Example

        assert(!PosInfInterval!Date(Date(1996, 1, 2)).intersects(
                    NegInfInterval!Date(Date(1996, 1, 2))));

        assert(PosInfInterval!Date(Date(1996, 1, 2)).intersects(
                    NegInfInterval!Date(Date(2000, 7, 1))));

  const Interval!TP `intersection`(in Interval!TP `interval`);
    Returns the `intersection` of two intervals
    Parameters:

    Interval!TP `interval` | The `interval` to intersect with this `interval`.

    Throws:
    std.datetime.date.DateTimeException if the two intervals do not intersect or if the given `interval` is empty.

    Example

        assert(PosInfInterval!Date(Date(1996, 1, 2)).intersection(
                    Interval!Date(Date(1990, 7, 6), Date(2000, 8, 2))) ==
               Interval!Date(Date(1996, 1 , 2), Date(2000, 8, 2)));

        assert(PosInfInterval!Date(Date(1996, 1, 2)).intersection(
                    Interval!Date(Date(1999, 1, 12), Date(2011, 9, 17))) ==
               Interval!Date(Date(1999, 1 , 12), Date(2011, 9, 17)));

  const pure nothrow PosInfInterval `intersection`(in PosInfInterval `interval`);
    Returns the `intersection` of two intervals
    Parameters:

    PosInfInterval `interval` | The `interval` to intersect with this `interval`.

    Example

        assert(PosInfInterval!Date(Date(1996, 1, 2)).intersection(
                    PosInfInterval!Date(Date(1990, 7, 6))) ==
               PosInfInterval!Date(Date(1996, 1 , 2)));

        assert(PosInfInterval!Date(Date(1996, 1, 2)).intersection(
                    PosInfInterval!Date(Date(1999, 1, 12))) ==
               PosInfInterval!Date(Date(1999, 1 , 12)));

  const Interval!TP `intersection`(in NegInfInterval!TP `interval`);
    Returns the `intersection` of two intervals
    Parameters:

    NegInfInterval!TP `interval` | The `interval` to intersect with this `interval`.

    Throws:
    std.datetime.date.DateTimeException if the two intervals do not intersect.

    Example

        assert(PosInfInterval!Date(Date(1996, 1, 2)).intersection(
                    NegInfInterval!Date(Date(1999, 7, 6))) ==
               Interval!Date(Date(1996, 1 , 2), Date(1999, 7, 6)));

        assert(PosInfInterval!Date(Date(1996, 1, 2)).intersection(
                    NegInfInterval!Date(Date(2013, 1, 12))) ==
               Interval!Date(Date(1996, 1 , 2), Date(2013, 1, 12)));

  const pure bool `isAdjacent`(in Interval!TP `interval`);
    Whether the given `interval` is adjacent to this `interval`.
    Parameters:

    Interval!TP `interval` | The `interval` to check whether its adjecent to this `interval`.

    Throws:
    std.datetime.date.DateTimeException if the given `interval` is empty.

    Example

        assert(PosInfInterval!Date(Date(1996, 1, 2)).isAdjacent(
                    Interval!Date(Date(1989, 3, 1), Date(1996, 1, 2))));

        assert(!PosInfInterval!Date(Date(1999, 1, 12)).isAdjacent(
                    Interval!Date(Date(1999, 1, 12), Date(2011, 9, 17))));

  const pure nothrow bool `isAdjacent`(in PosInfInterval `interval`);
    Whether the given `interval` is adjacent to this `interval`.
    Always returns `false` because two intervals going to positive infinity can never be adjacent to one another.
    Parameters:

    PosInfInterval `interval` | The `interval` to check whether its adjecent to this `interval`.

    Example

        assert(!PosInfInterval!Date(Date(1996, 1, 2)).isAdjacent(
                    PosInfInterval!Date(Date(1990, 1, 7))));

        assert(!PosInfInterval!Date(Date(1996, 1, 2)).isAdjacent(
                    PosInfInterval!Date(Date(1996, 1, 2))));

  const pure nothrow bool `isAdjacent`(in NegInfInterval!TP `interval`);
    Whether the given `interval` is adjacent to this `interval`.
    Parameters:

    NegInfInterval!TP `interval` | The `interval` to check whether its adjecent to this `interval`.

    Example

        assert(PosInfInterval!Date(Date(1996, 1, 2)).isAdjacent(
                    NegInfInterval!Date(Date(1996, 1, 2))));

        assert(!PosInfInterval!Date(Date(1996, 1, 2)).isAdjacent(
                    NegInfInterval!Date(Date(2000, 7, 1))));

  const PosInfInterval `merge`(in Interval!TP `interval`);
    Returns the union of two intervals
    Parameters:

    Interval!TP `interval` | The `interval` to `merge` with this `interval`.

    Throws:
    std.datetime.date.DateTimeException if the two intervals do not intersect and are not adjacent or if the given `interval` is empty.

    Note There is no overload for `merge` which takes a NegInfInterval, because an `interval` going from negative infinity to positive infinity is not possible.

    Example

        assert(PosInfInterval!Date(Date(1996, 1, 2)).merge(
                    Interval!Date(Date(1990, 7, 6), Date(2000, 8, 2))) ==
               PosInfInterval!Date(Date(1990, 7 , 6)));

        assert(PosInfInterval!Date(Date(1996, 1, 2)).merge(
                    Interval!Date(Date(1999, 1, 12), Date(2011, 9, 17))) ==
               PosInfInterval!Date(Date(1996, 1 , 2)));

  const pure nothrow PosInfInterval `merge`(in PosInfInterval `interval`);
    Returns the union of two intervals
    Parameters:

    PosInfInterval `interval` | The `interval` to `merge` with this `interval`.

    Note There is no overload for `merge` which takes a NegInfInterval, because an `interval` going from negative infinity to positive infinity is not possible.

    Example

        assert(PosInfInterval!Date(Date(1996, 1, 2)).merge(
                    PosInfInterval!Date(Date(1990, 7, 6))) ==
               PosInfInterval!Date(Date(1990, 7 , 6)));

        assert(PosInfInterval!Date(Date(1996, 1, 2)).merge(
                    PosInfInterval!Date(Date(1999, 1, 12))) ==
               PosInfInterval!Date(Date(1996, 1 , 2)));

  const pure PosInfInterval `span`(in Interval!TP `interval`);
    Returns an `interval` that covers from the earliest time point of two intervals up to (but not including) the latest time point of two intervals.
    Parameters:

    Interval!TP `interval` | The `interval` to create a `span` together with this `interval`.

    Throws:
    std.datetime.date.DateTimeException if the given `interval` is empty.

    Note There is no overload for `span` which takes a NegInfInterval, because an `interval` going from negative infinity to positive infinity is not possible.

    Example

        assert(PosInfInterval!Date(Date(1996, 1, 2)).span(
                    Interval!Date(Date(500, 8, 9), Date(1602, 1, 31))) ==
               PosInfInterval!Date(Date(500, 8, 9)));

        assert(PosInfInterval!Date(Date(1996, 1, 2)).span(
                    Interval!Date(Date(1990, 7, 6), Date(2000, 8, 2))) ==
               PosInfInterval!Date(Date(1990, 7 , 6)));

        assert(PosInfInterval!Date(Date(1996, 1, 2)).span(
                    Interval!Date(Date(1999, 1, 12), Date(2011, 9, 17))) ==
               PosInfInterval!Date(Date(1996, 1 , 2)));

  const pure nothrow PosInfInterval `span`(in PosInfInterval `interval`);
    Returns an `interval` that covers from the earliest time point of two intervals up to (but not including) the latest time point of two intervals.
    Parameters:

    PosInfInterval `interval` | The `interval` to create a `span` together with this `interval`.

    Note There is no overload for `span` which takes a NegInfInterval, because an `interval` going from negative infinity to positive infinity is not possible.

    Example

        assert(PosInfInterval!Date(Date(1996, 1, 2)).span(
                    PosInfInterval!Date(Date(1990, 7, 6))) ==
               PosInfInterval!Date(Date(1990, 7 , 6)));

        assert(PosInfInterval!Date(Date(1996, 1, 2)).span(
                    PosInfInterval!Date(Date(1999, 1, 12))) ==
               PosInfInterval!Date(Date(1996, 1 , 2)));

  pure nothrow void `shift`(D)(D `duration`)
  if (__traits(compiles, begin + `duration`));
    Shifts the begin of this interval forward or backwards in time by the given `duration` (a positive `duration` shifts the interval forward; a negative `duration` shifts it backward). Effectively, it does begin += `duration`.
    Parameters:

    D `duration` | The `duration` to `shift` the interval by.

    Example

        auto interval1 = PosInfInterval!Date(Date(1996, 1, 2));
        auto interval2 = PosInfInterval!Date(Date(1996, 1, 2));

        interval1.shift(dur!"days"(50));
        assert(interval1 == PosInfInterval!Date(Date(1996, 2, 21)));

        interval2.shift(dur!"days"(-50));
        assert(interval2 == PosInfInterval!Date(Date(1995, 11, 13)));

  void `shift`(T)(T `years`, T `months` = 0, AllowDayOverflow `allowOverflow` = AllowDayOverflow.yes)
  if (isIntegral!T);
    Shifts the begin of this interval forward or backwards in time by the given number of `years` and/or `months` (a positive number of `years` and `months` shifts the interval forward; a negative number shifts it backward). It adds the `years` the given `years` and `months` to begin. It effectively calls add!"`years`"() and then add!"`months`"() on begin with the given number of `years` and `months`.
    Parameters:

    T `years` | The number of `years` to `shift` the interval by.
    T `months` | The number of `months` to `shift` the interval by.
    AllowDayOverflow `allowOverflow` | Whether the days should be allowed to overflow on begin, causing its month to increment.

    Throws:
    std.datetime.date.DateTimeException if this interval is empty or if the resulting interval would be invalid.

    Example

        auto interval1 = PosInfInterval!Date(Date(1996, 1, 2));
        auto interval2 = PosInfInterval!Date(Date(1996, 1, 2));

        interval1.shift(dur!"days"(50));
        assert(interval1 == PosInfInterval!Date(Date(1996, 2, 21)));

        interval2.shift(dur!"days"(-50));
        assert(interval2 == PosInfInterval!Date(Date(1995, 11, 13)));

  pure nothrow void `expand`(D)(D `duration`)
  if (__traits(compiles, begin + `duration`));
    Expands the interval backwards in time. Effectively, it does begin -= `duration`.
    Parameters:

    D `duration` | The `duration` to `expand` the interval by.

    Example

        auto interval1 = PosInfInterval!Date(Date(1996, 1, 2));
        auto interval2 = PosInfInterval!Date(Date(1996, 1, 2));

        interval1.expand(dur!"days"(2));
        assert(interval1 == PosInfInterval!Date(Date(1995, 12, 31)));

        interval2.expand(dur!"days"(-2));
        assert(interval2 == PosInfInterval!Date(Date(1996, 1, 4)));

  void `expand`(T)(T `years`, T `months` = 0, AllowDayOverflow `allowOverflow` = AllowDayOverflow.yes)
  if (isIntegral!T);
    Expands the interval forwards and/or backwards in time. Effectively, it subtracts the given number of `months`/`years` from begin.
    Parameters:

    T `years` | The number of `years` to `expand` the interval by.
    T `months` | The number of `months` to `expand` the interval by.
    AllowDayOverflow `allowOverflow` | Whether the days should be allowed to overflow on begin, causing its month to increment.

    Throws:
    std.datetime.date.DateTimeException if this interval is empty or if the resulting interval would be invalid.

    Example

        auto interval1 = PosInfInterval!Date(Date(1996, 1, 2));
        auto interval2 = PosInfInterval!Date(Date(1996, 1, 2));

        interval1.expand(2);
        assert(interval1 == PosInfInterval!Date(Date(1994, 1, 2)));

        interval2.expand(-2);
        assert(interval2 == PosInfInterval!Date(Date(1998, 1, 2)));

  const PosInfIntervalRange!TP `fwdRange`(TP delegate(in TP) `func`, PopFirst `popFirst` = PopFirst.no);
    Returns a range which iterates forward over the interval, starting at begin, using `func` to generate each successive time point.
    The range's front is the interval's begin. `func` is used to generate the next front when popFront is called. If `popFirst` is PopFirst.yes, then popFront is called before the range is returned (so that front is a time point which `func` would generate).
    If `func` ever generates a time point less than or equal to the current front of the range, then a std.datetime.date.DateTimeException will be thrown.
    There are helper functions in this module which generate common delegates to pass to `fwdRange`. Their documentation starts with "Range-generating function," to make them easily searchable.
    Parameters:

    TP delegate(in TP) `func` | The function used to generate the time points of the range over the interval.
    PopFirst `popFirst` | Whether popFront should be called on the range before returning it.

    Throws:
    std.datetime.date.DateTimeException if this interval is empty.

    Warning `func` must be logically pure. Ideally, `func` would be a function pointer to a pure function, but forcing `func` to be pure is far too restrictive to be useful, and in order to have the ease of use of having functions which generate functions to pass to `fwdRange`, `func` must be a delegate.

    If `func` retains state which changes as it is called, then some algorithms will not work correctly, because the range's save will have failed to have really saved the range's state. To avoid such bugs, don't pass a delegate which is not logically pure to `fwdRange`. If `func` is given the same time point with two different calls, it must return the same result both times.
    Of course, none of the functions in this module have this problem, so it's only relevant for custom delegates.

    Example

        auto interval = PosInfInterval!Date(Date(2010, 9, 1));
        auto func = delegate (in Date date) //For iterating over even-numbered days.
                    {
                        if ((date.day & 1) == 0)
                            return date + dur!"days"(2);

                        return date + dur!"days"(1);
                    };
        auto range = interval.fwdRange(func);

        //An odd day. Using PopFirst.yes would have made this Date(2010, 9, 2).
        assert(range.front == Date(2010, 9, 1));

        range.popFront();
        assert(range.front == Date(2010, 9, 2));

        range.popFront();
        assert(range.front == Date(2010, 9, 4));

        range.popFront();
        assert(range.front == Date(2010, 9, 6));

        range.popFront();
        assert(range.front == Date(2010, 9, 8));

        range.popFront();
        assert(!range.empty);

  const nothrow string `toString`();
    Converts this interval to a string.
struct `NegInfInterval`(TP);
  Represents an interval of time which has negative infinity as its starting point.
  Any ranges which iterate over a `NegInfInterval` are infinite. So, the main purpose of using `NegInfInterval` is to create an infinite range which starts at negative infinity and goes to a fixed end point. Iterate over it in reverse.
  pure nothrow this(in TP `end`);
    Parameters:

    TP `end` | The time point which ends the interval.

    Example

        auto interval = PosInfInterval!Date(Date(1996, 1, 2));

  pure nothrow ref NegInfInterval `opAssign`(ref const NegInfInterval `rhs`);
    Parameters:

    NegInfInterval `rhs` | The NegInfInterval to assign to this one.

  pure nothrow ref NegInfInterval `opAssign`(NegInfInterval `rhs`);
    Parameters:

    NegInfInterval `rhs` | The NegInfInterval to assign to this one.

  const pure nothrow @property TP `end`();
    The `end` point of the interval. It is excluded from the interval.

    Example

        assert(NegInfInterval!Date(Date(2012, 3, 1)).end == Date(2012, 3, 1));

  pure nothrow @property void `end`(TP `timePoint`);
    The `end` point of the interval. It is excluded from the interval.
    Parameters:

    TP `timePoint` | The time point to set `end` to.

  enum bool `empty`;
    Whether the interval's length is 0. Always returns `false`.

    Example

        assert(!NegInfInterval!Date(Date(1996, 1, 2)).empty);

  const pure nothrow bool `contains`(TP `timePoint`);
    Whether the given time point is within this interval.
    Parameters:

    TP `timePoint` | The time point to check for inclusion in this interval.

    Example

        assert(NegInfInterval!Date(Date(2012, 3, 1)).contains(Date(1994, 12, 24)));
        assert(NegInfInterval!Date(Date(2012, 3, 1)).contains(Date(2000, 1, 5)));
        assert(!NegInfInterval!Date(Date(2012, 3, 1)).contains(Date(2012, 3, 1)));

  const pure bool `contains`(in Interval!TP `interval`);
    Whether the given `interval` is completely within this `interval`.
    Parameters:

    Interval!TP `interval` | The `interval` to check for inclusion in this `interval`.

    Throws:
    std.datetime.date.DateTimeException if the given `interval` is empty.

    Example

        assert(NegInfInterval!Date(Date(2012, 3, 1)).contains(
                    Interval!Date(Date(1990, 7, 6), Date(2000, 8, 2))));

        assert(NegInfInterval!Date(Date(2012, 3, 1)).contains(
                    Interval!Date(Date(1999, 1, 12), Date(2011, 9, 17))));

        assert(!NegInfInterval!Date(Date(2012, 3, 1)).contains(
                    Interval!Date(Date(1998, 2, 28), Date(2013, 5, 1))));

  const pure nothrow bool `contains`(in PosInfInterval!TP `interval`);
    Whether the given `interval` is completely within this `interval`.
    Always returns `false` because an `interval` beginning at negative infinity can never contain an `interval` going to positive infinity.
    Parameters:

    PosInfInterval!TP `interval` | The `interval` to check for inclusion in this `interval`.

    Example

        assert(!NegInfInterval!Date(Date(2012, 3, 1)).contains(
                    PosInfInterval!Date(Date(1999, 5, 4))));

  const pure nothrow bool `contains`(in NegInfInterval `interval`);
    Whether the given `interval` is completely within this `interval`.
    Parameters:

    NegInfInterval `interval` | The `interval` to check for inclusion in this `interval`.

    Example

        assert(NegInfInterval!Date(Date(2012, 3, 1)).contains(
                    NegInfInterval!Date(Date(1996, 5, 4))));

        assert(!NegInfInterval!Date(Date(2012, 3, 1)).contains(
                    NegInfInterval!Date(Date(2013, 7, 9))));

  const pure nothrow bool `isBefore`(in TP `timePoint`);
    Whether this interval is before the given time point.
    Parameters:

    TP `timePoint` | The time point to check whether this interval is before it.

    Example

        assert(!NegInfInterval!Date(Date(2012, 3, 1)).isBefore(Date(1994, 12, 24)));
        assert(!NegInfInterval!Date(Date(2012, 3, 1)).isBefore(Date(2000, 1, 5)));
        assert(NegInfInterval!Date(Date(2012, 3, 1)).isBefore(Date(2012, 3, 1)));

  const pure bool `isBefore`(in Interval!TP `interval`);
    Whether this `interval` is before the given `interval` and does not intersect it.
    Parameters:

    Interval!TP `interval` | The `interval` to check for against this `interval`.

    Throws:
    std.datetime.date.DateTimeException if the given `interval` is empty

    Example

        assert(!NegInfInterval!Date(Date(2012, 3, 1)).isBefore(
                    Interval!Date(Date(1990, 7, 6), Date(2000, 8, 2))));

        assert(!NegInfInterval!Date(Date(2012, 3, 1)).isBefore(
                    Interval!Date(Date(1999, 1, 12), Date(2011, 9, 17))));

        assert(NegInfInterval!Date(Date(2012, 3, 1)).isBefore(
                    Interval!Date(Date(2022, 10, 19), Date(2027, 6, 3))));

  const pure nothrow bool `isBefore`(in PosInfInterval!TP `interval`);
    Whether this `interval` is before the given `interval` and does not intersect it.
    Parameters:

    PosInfInterval!TP `interval` | The `interval` to check for against this `interval`.

    Example

        assert(!NegInfInterval!Date(Date(2012, 3, 1)).isBefore(
                    PosInfInterval!Date(Date(1999, 5, 4))));

        assert(NegInfInterval!Date(Date(2012, 3, 1)).isBefore(
                    PosInfInterval!Date(Date(2012, 3, 1))));

  const pure nothrow bool `isBefore`(in NegInfInterval `interval`);
    Whether this `interval` is before the given `interval` and does not intersect it.
    Always returns `false` because an `interval` beginning at negative infinity can never be before another `interval` beginning at negative infinity.
    Parameters:

    NegInfInterval `interval` | The `interval` to check for against this `interval`.

    Example

        assert(!NegInfInterval!Date(Date(2012, 3, 1)).isBefore(
                    NegInfInterval!Date(Date(1996, 5, 4))));

        assert(!NegInfInterval!Date(Date(2012, 3, 1)).isBefore(
                    NegInfInterval!Date(Date(2013, 7, 9))));

  const pure nothrow bool `isAfter`(in TP `timePoint`);
    Whether this interval is after the given time point.
    Always returns `false` because an interval beginning at negative infinity can never be after any time point.
    Parameters:

    TP `timePoint` | The time point to check whether this interval is after it.

    Example

        assert(!NegInfInterval!Date(Date(2012, 3, 1)).isAfter(Date(1994, 12, 24)));
        assert(!NegInfInterval!Date(Date(2012, 3, 1)).isAfter(Date(2000, 1, 5)));
        assert(!NegInfInterval!Date(Date(2012, 3, 1)).isAfter(Date(2012, 3, 1)));

  const pure bool `isAfter`(in Interval!TP `interval`);
    Whether this `interval` is after the given `interval` and does not intersect it.
    Always returns `false` (unless the given `interval` is empty) because an `interval` beginning at negative infinity can never be after any other `interval`.
    Parameters:

    Interval!TP `interval` | The `interval` to check against this `interval`.

    Throws:
    std.datetime.date.DateTimeException if the given `interval` is empty.

    Example

        assert(!NegInfInterval!Date(Date(2012, 3, 1)).isAfter(
                    Interval!Date(Date(1990, 7, 6), Date(2000, 8, 2))));

        assert(!NegInfInterval!Date(Date(2012, 3, 1)).isAfter(
                    Interval!Date(Date(1999, 1, 12), Date(2011, 9, 17))));

        assert(!NegInfInterval!Date(Date(2012, 3, 1)).isAfter(
                    Interval!Date(Date(2022, 10, 19), Date(2027, 6, 3))));

  const pure nothrow bool `isAfter`(in PosInfInterval!TP `interval`);
    Whether this `interval` is after the given `interval` and does not intersect it.
    Always returns `false` because an `interval` beginning at negative infinity can never be after any other `interval`.
    Parameters:

    PosInfInterval!TP `interval` | The `interval` to check against this `interval`.

    Example

        assert(!NegInfInterval!Date(Date(2012, 3, 1)).isAfter(
                    PosInfInterval!Date(Date(1999, 5, 4))));

        assert(!NegInfInterval!Date(Date(2012, 3, 1)).isAfter(
                    PosInfInterval!Date(Date(2012, 3, 1))));

  const pure nothrow bool `isAfter`(in NegInfInterval `interval`);
    Whether this `interval` is after the given `interval` and does not intersect it.
    Always returns `false` because an `interval` beginning at negative infinity can never be after any other `interval`.
    Parameters:

    NegInfInterval `interval` | The `interval` to check against this `interval`.

    Example

        assert(!NegInfInterval!Date(Date(2012, 3, 1)).isAfter(
                    NegInfInterval!Date(Date(1996, 5, 4))));

        assert(!NegInfInterval!Date(Date(2012, 3, 1)).isAfter(
                    NegInfInterval!Date(Date(2013, 7, 9))));

  const pure bool `intersects`(in Interval!TP `interval`);
    Whether the given `interval` overlaps this `interval`.
    Parameters:

    Interval!TP `interval` | The `interval` to check for intersection with this `interval`.

    Throws:
    std.datetime.date.DateTimeException if the given `interval` is empty.

    Example

        assert(NegInfInterval!Date(Date(2012, 3, 1)).intersects(
                    Interval!Date(Date(1990, 7, 6), Date(2000, 8, 2))));

        assert(NegInfInterval!Date(Date(2012, 3, 1)).intersects(
                    Interval!Date(Date(1999, 1, 12), Date(2011, 9, 17))));

        assert(!NegInfInterval!Date(Date(2012, 3, 1)).intersects(
                    Interval!Date(Date(2022, 10, 19), Date(2027, 6, 3))));

  const pure nothrow bool `intersects`(in PosInfInterval!TP `interval`);
    Whether the given `interval` overlaps this `interval`.
    Parameters:

    PosInfInterval!TP `interval` | The `interval` to check for intersection with this `interval`.

    Example

        assert(NegInfInterval!Date(Date(2012, 3, 1)).intersects(
                    PosInfInterval!Date(Date(1999, 5, 4))));

        assert(!NegInfInterval!Date(Date(2012, 3, 1)).intersects(
                    PosInfInterval!Date(Date(2012, 3, 1))));

  const pure nothrow bool `intersects`(in NegInfInterval!TP `interval`);
    Whether the given `interval` overlaps this `interval`.
    Always returns `true` because two intervals beginning at negative infinity always overlap.
    Parameters:

    NegInfInterval!TP `interval` | The `interval` to check for intersection with this `interval`.

    Example

        assert(NegInfInterval!Date(Date(2012, 3, 1)).intersects(
                    NegInfInterval!Date(Date(1996, 5, 4))));

        assert(NegInfInterval!Date(Date(2012, 3, 1)).intersects(
                    NegInfInterval!Date(Date(2013, 7, 9))));

  const Interval!TP `intersection`(in Interval!TP `interval`);
    Returns the `intersection` of two intervals
    Parameters:

    Interval!TP `interval` | The `interval` to intersect with this `interval`.

    Throws:
    std.datetime.date.DateTimeException if the two intervals do not intersect or if the given `interval` is empty.

    Example

        assert(NegInfInterval!Date(Date(2012, 3, 1)).intersection(
                    Interval!Date(Date(1990, 7, 6), Date(2000, 8, 2))) ==
               Interval!Date(Date(1990, 7 , 6), Date(2000, 8, 2)));

        assert(NegInfInterval!Date(Date(2012, 3, 1)).intersection(
                    Interval!Date(Date(1999, 1, 12), Date(2015, 9, 2))) ==
               Interval!Date(Date(1999, 1 , 12), Date(2012, 3, 1)));

  const Interval!TP `intersection`(in PosInfInterval!TP `interval`);
    Returns the `intersection` of two intervals
    Parameters:

    PosInfInterval!TP `interval` | The `interval` to intersect with this `interval`.

    Throws:
    std.datetime.date.DateTimeException if the two intervals do not intersect.

    Example

        assert(NegInfInterval!Date(Date(2012, 3, 1)).intersection(
                    PosInfInterval!Date(Date(1990, 7, 6))) ==
               Interval!Date(Date(1990, 7 , 6), Date(2012, 3, 1)));

        assert(NegInfInterval!Date(Date(2012, 3, 1)).intersection(
                    PosInfInterval!Date(Date(1999, 1, 12))) ==
               Interval!Date(Date(1999, 1 , 12), Date(2012, 3, 1)));

  const nothrow NegInfInterval `intersection`(in NegInfInterval `interval`);
    Returns the `intersection` of two intervals
    Parameters:

    NegInfInterval `interval` | The `interval` to intersect with this `interval`.

    Example

        assert(NegInfInterval!Date(Date(2012, 3, 1)).intersection(
                    NegInfInterval!Date(Date(1999, 7, 6))) ==
               NegInfInterval!Date(Date(1999, 7 , 6)));

        assert(NegInfInterval!Date(Date(2012, 3, 1)).intersection(
                    NegInfInterval!Date(Date(2013, 1, 12))) ==
               NegInfInterval!Date(Date(2012, 3 , 1)));

  const pure bool `isAdjacent`(in Interval!TP `interval`);
    Whether the given `interval` is adjacent to this `interval`.
    Parameters:

    Interval!TP `interval` | The `interval` to check whether its adjecent to this `interval`.

    Throws:
    std.datetime.date.DateTimeException if the given `interval` is empty.

    Example

        assert(!NegInfInterval!Date(Date(2012, 3, 1)).isAdjacent(
                    Interval!Date(Date(1990, 7, 6), Date(2000, 8, 2))));

        assert(!NegInfInterval!Date(Date(2012, 3, 1)).isAdjacent(
                    Interval!Date(Date(1999, 1, 12), Date(2012, 3, 1))));

        assert(NegInfInterval!Date(Date(2012, 3, 1)).isAdjacent(
                    Interval!Date(Date(2012, 3, 1), Date(2019, 2, 2))));

        assert(!NegInfInterval!Date(Date(2012, 3, 1)).isAdjacent(
                    Interval!Date(Date(2022, 10, 19), Date(2027, 6, 3))));

  const pure nothrow bool `isAdjacent`(in PosInfInterval!TP `interval`);
    Whether the given `interval` is adjacent to this `interval`.
    Parameters:

    PosInfInterval!TP `interval` | The `interval` to check whether its adjecent to this `interval`.

    Example

        assert(!NegInfInterval!Date(Date(2012, 3, 1)).isAdjacent(
                    PosInfInterval!Date(Date(1999, 5, 4))));

        assert(NegInfInterval!Date(Date(2012, 3, 1)).isAdjacent(
                    PosInfInterval!Date(Date(2012, 3, 1))));

  const pure nothrow bool `isAdjacent`(in NegInfInterval `interval`);
    Whether the given `interval` is adjacent to this `interval`.
    Always returns `false` because two intervals beginning at negative infinity can never be adjacent to one another.
    Parameters:

    NegInfInterval `interval` | The `interval` to check whether its adjecent to this `interval`.

    Example

        assert(!NegInfInterval!Date(Date(2012, 3, 1)).isAdjacent(
                    NegInfInterval!Date(Date(1996, 5, 4))));

        assert(!NegInfInterval!Date(Date(2012, 3, 1)).isAdjacent(
                    NegInfInterval!Date(Date(2012, 3, 1))));

  const NegInfInterval `merge`(in Interval!TP `interval`);
    Returns the union of two intervals
    Parameters:

    Interval!TP `interval` | The `interval` to `merge` with this `interval`.

    Throws:
    std.datetime.date.DateTimeException if the two intervals do not intersect and are not adjacent or if the given `interval` is empty.

    Note There is no overload for `merge` which takes a PosInfInterval, because an `interval` going from negative infinity to positive infinity is not possible.

    Example

        assert(NegInfInterval!Date(Date(2012, 3, 1)).merge(
                    Interval!Date(Date(1990, 7, 6), Date(2000, 8, 2))) ==
               NegInfInterval!Date(Date(2012, 3 , 1)));

        assert(NegInfInterval!Date(Date(2012, 3, 1)).merge(
                    Interval!Date(Date(1999, 1, 12), Date(2015, 9, 2))) ==
               NegInfInterval!Date(Date(2015, 9 , 2)));

  const pure nothrow NegInfInterval `merge`(in NegInfInterval `interval`);
    Returns the union of two intervals
    Parameters:

    NegInfInterval `interval` | The `interval` to `merge` with this `interval`.

    Note There is no overload for `merge` which takes a PosInfInterval, because an `interval` going from negative infinity to positive infinity is not possible.

    Example

        assert(NegInfInterval!Date(Date(2012, 3, 1)).merge(
                    NegInfInterval!Date(Date(1999, 7, 6))) ==
               NegInfInterval!Date(Date(2012, 3 , 1)));

        assert(NegInfInterval!Date(Date(2012, 3, 1)).merge(
                    NegInfInterval!Date(Date(2013, 1, 12))) ==
               NegInfInterval!Date(Date(2013, 1 , 12)));

  const pure NegInfInterval `span`(in Interval!TP `interval`);
    Returns an `interval` that covers from the earliest time point of two intervals up to (but not including) the latest time point of two intervals.
    Parameters:

    Interval!TP `interval` | The `interval` to create a `span` together with this `interval`.

    Throws:
    std.datetime.date.DateTimeException if the given `interval` is empty.

    Note There is no overload for `span` which takes a PosInfInterval, because an `interval` going from negative infinity to positive infinity is not possible.

    Example

        assert(NegInfInterval!Date(Date(2012, 3, 1)).span(
                    Interval!Date(Date(1990, 7, 6), Date(2000, 8, 2))) ==
               NegInfInterval!Date(Date(2012, 3 , 1)));

        assert(NegInfInterval!Date(Date(2012, 3, 1)).span(
                    Interval!Date(Date(1999, 1, 12), Date(2015, 9, 2))) ==
               NegInfInterval!Date(Date(2015, 9 , 2)));

        assert(NegInfInterval!Date(Date(1600, 1, 7)).span(
                    Interval!Date(Date(2012, 3, 11), Date(2017, 7, 1))) ==
               NegInfInterval!Date(Date(2017, 7 , 1)));

  const pure nothrow NegInfInterval `span`(in NegInfInterval `interval`);
    Returns an `interval` that covers from the earliest time point of two intervals up to (but not including) the latest time point of two intervals.
    Parameters:

    NegInfInterval `interval` | The `interval` to create a `span` together with this `interval`.

    Note There is no overload for `span` which takes a PosInfInterval, because an `interval` going from negative infinity to positive infinity is not possible.

    Example

        assert(NegInfInterval!Date(Date(2012, 3, 1)).span(
                    NegInfInterval!Date(Date(1999, 7, 6))) ==
               NegInfInterval!Date(Date(2012, 3 , 1)));

        assert(NegInfInterval!Date(Date(2012, 3, 1)).span(
                    NegInfInterval!Date(Date(2013, 1, 12))) ==
               NegInfInterval!Date(Date(2013, 1 , 12)));

  pure nothrow void `shift`(D)(D `duration`)
  if (__traits(compiles, end + `duration`));
    Shifts the end of this interval forward or backwards in time by the given `duration` (a positive `duration` shifts the interval forward; a negative `duration` shifts it backward). Effectively, it does end += `duration`.
    Parameters:

    D `duration` | The `duration` to `shift` the interval by.

    Example

        auto interval1 = NegInfInterval!Date(Date(2012, 4, 5));
        auto interval2 = NegInfInterval!Date(Date(2012, 4, 5));

        interval1.shift(dur!"days"(50));
        assert(interval1 == NegInfInterval!Date(Date(2012, 5, 25)));

        interval2.shift(dur!"days"(-50));
        assert(interval2 == NegInfInterval!Date( Date(2012, 2, 15)));

  void `shift`(T)(T `years`, T `months` = 0, AllowDayOverflow `allowOverflow` = AllowDayOverflow.yes)
  if (isIntegral!T);
    Shifts the end of this interval forward or backwards in time by the given number of `years` and/or `months` (a positive number of `years` and `months` shifts the interval forward; a negative number shifts it backward). It adds the `years` the given `years` and `months` to end. It effectively calls add!"`years`"() and then add!"`months`"() on end with the given number of `years` and `months`.
    Parameters:

    T `years` | The number of `years` to `shift` the interval by.
    T `months` | The number of `months` to `shift` the interval by.
    AllowDayOverflow `allowOverflow` | Whether the days should be allowed to overflow on end, causing its month to increment.

    Throws:
    std.datetime.date.DateTimeException if empty is `true` or if the resulting interval would be invalid.

    Example

        auto interval1 = NegInfInterval!Date(Date(2012, 3, 1));
        auto interval2 = NegInfInterval!Date(Date(2012, 3, 1));

        interval1.shift(2);
        assert(interval1 == NegInfInterval!Date(Date(2014, 3, 1)));

        interval2.shift(-2);
        assert(interval2 == NegInfInterval!Date(Date(2010, 3, 1)));

  pure nothrow void `expand`(D)(D `duration`)
  if (__traits(compiles, end + `duration`));
    Expands the interval forwards in time. Effectively, it does end += `duration`.
    Parameters:

    D `duration` | The `duration` to `expand` the interval by.

    Example

        auto interval1 = NegInfInterval!Date(Date(2012, 3, 1));
        auto interval2 = NegInfInterval!Date(Date(2012, 3, 1));

        interval1.expand(dur!"days"(2));
        assert(interval1 == NegInfInterval!Date(Date(2012, 3, 3)));

        interval2.expand(dur!"days"(-2));
        assert(interval2 == NegInfInterval!Date(Date(2012, 2, 28)));

  void `expand`(T)(T `years`, T `months` = 0, AllowDayOverflow `allowOverflow` = AllowDayOverflow.yes)
  if (isIntegral!T);
    Expands the interval forwards and/or backwards in time. Effectively, it adds the given number of `months`/`years` to end.
    Parameters:

    T `years` | The number of `years` to `expand` the interval by.
    T `months` | The number of `months` to `expand` the interval by.
    AllowDayOverflow `allowOverflow` | Whether the days should be allowed to overflow on end, causing their month to increment.

    Throws:
    std.datetime.date.DateTimeException if empty is `true` or if the resulting interval would be invalid.

    Example

        auto interval1 = NegInfInterval!Date(Date(2012, 3, 1));
        auto interval2 = NegInfInterval!Date(Date(2012, 3, 1));

        interval1.expand(2);
        assert(interval1 == NegInfInterval!Date(Date(2014, 3, 1)));

        interval2.expand(-2);
        assert(interval2 == NegInfInterval!Date(Date(2010, 3, 1)));

  const NegInfIntervalRange!TP `bwdRange`(TP delegate(in TP) `func`, PopFirst `popFirst` = PopFirst.no);
    Returns a range which iterates backwards over the interval, starting at end, using `func` to generate each successive time point.
    The range's front is the interval's end. `func` is used to generate the next front when popFront is called. If `popFirst` is PopFirst.yes, then popFront is called before the range is returned (so that front is a time point which `func` would generate).
    If `func` ever generates a time point greater than or equal to the current front of the range, then a std.datetime.date.DateTimeException will be thrown.
    There are helper functions in this module which generate common delegates to pass to `bwdRange`. Their documentation starts with "Range-generating function," to make them easily searchable.
    Parameters:

    TP delegate(in TP) `func` | The function used to generate the time points of the range over the interval.
    PopFirst `popFirst` | Whether popFront should be called on the range before returning it.

    Throws:
    std.datetime.date.DateTimeException if this interval is empty.

    Warning `func` must be logically pure. Ideally, `func` would be a function pointer to a pure function, but forcing `func` to be pure is far too restrictive to be useful, and in order to have the ease of use of having functions which generate functions to pass to fwdRange, `func` must be a delegate.

    If `func` retains state which changes as it is called, then some algorithms will not work correctly, because the range's save will have failed to have really saved the range's state. To avoid such bugs, don't pass a delegate which is not logically pure to fwdRange. If `func` is given the same time point with two different calls, it must return the same result both times.
    Of course, none of the functions in this module have this problem, so it's only relevant for custom delegates.

    Example

        auto interval = NegInfInterval!Date(Date(2010, 9, 9));
        auto func = delegate (in Date date) //For iterating over even-numbered days.
                    {
                        if ((date.day & 1) == 0)
                            return date - dur!"days"(2);

                        return date - dur!"days"(1);
                    };
        auto range = interval.bwdRange(func);

        assert(range.front == Date(2010, 9, 9)); //An odd day. Using PopFirst.yes would have made this Date(2010, 9, 8).

        range.popFront();
        assert(range.front == Date(2010, 9, 8));

        range.popFront();
        assert(range.front == Date(2010, 9, 6));

        range.popFront();
        assert(range.front == Date(2010, 9, 4));

        range.popFront();
        assert(range.front == Date(2010, 9, 2));

        range.popFront();
        assert(!range.empty);

  const nothrow string `toString`();
    Converts this interval to a string.
nothrow TP delegate(in TP) `everyDayOfWeek`(TP, Direction dir = Direction.fwd)(DayOfWeek `dayOfWeek`)
if (isTimePoint!TP && (dir == Direction.fwd || dir == Direction.bwd) && __traits(hasMember, TP, "`dayOfWeek`") && !__traits(isStaticFunction, TP.`dayOfWeek`) && is(typeof(TP.`dayOfWeek`) == DayOfWeek));
  Range-generating function.
  Returns a delegate which returns the next time point with the given DayOfWeek in a range.
  Using this delegate allows iteration over successive time points which are all the same day of the week. e.g. passing DayOfWeek.mon to `everyDayOfWeek` would result in a delegate which could be used to iterate over all of the Mondays in a range.
  Parameters:

  dir | The direction to iterate in. If passing the return value to fwdRange, use Direction.fwd. If passing it to bwdRange, use Direction.bwd.
  DayOfWeek `dayOfWeek` | The week that each time point in the range will be.

  Examples:

      import std.datetime.date : Date, DayOfWeek;

      auto interval = Interval!Date(Date(2010, 9, 2), Date(2010, 9, 27));
      auto func = everyDayOfWeek!Date(DayOfWeek.mon);
      auto range = interval.fwdRange(func);

      // A Thursday. Using PopFirst.yes would have made this Date(2010, 9, 6).
      writeln(range.front); // Date(2010, 9, 2)

      range.popFront();
      writeln(range.front); // Date(2010, 9, 6)

      range.popFront();
      writeln(range.front); // Date(2010, 9, 13)

      range.popFront();
      writeln(range.front); // Date(2010, 9, 20)

      range.popFront();
      assert(range.empty);

TP delegate(in TP) `everyMonth`(TP, Direction dir = Direction.fwd)(int `month`)
if (isTimePoint!TP && (dir == Direction.fwd || dir == Direction.bwd) && __traits(hasMember, TP, "`month`") && !__traits(isStaticFunction, TP.`month`) && is(typeof(TP.`month`) == Month));
  Range-generating function.
  Returns a delegate which returns the next time point with the given `month` which would be reached by adding months to the given time point.
  So, using this delegate allows iteration over successive time points which are in the same `month` but different years. For example, iterate over each successive December 25th in an interval by starting with a date which had the 25th as its day and passed Month.dec to `everyMonth` to create the delegate.
  Since it wouldn't really make sense to be iterating over a specific `month` and end up with some of the time points in the succeeding `month` or two years after the previous time point, AllowDayOverflow.no is always used when calculating the next time point.
  Parameters:

  dir | The direction to iterate in. If passing the return value to fwdRange, use Direction.fwd. If passing it to bwdRange, use Direction.bwd.
  int `month` | The `month` that each time point in the range will be in (January is 1).

  Examples:

      import std.datetime.date : Date, Month;

      auto interval = Interval!Date(Date(2000, 1, 30), Date(2004, 8, 5));
      auto func = everyMonth!Date(Month.feb);
      auto range = interval.fwdRange(func);

      // Using PopFirst.yes would have made this Date(2010, 2, 29).
      writeln(range.front); // Date(2000, 1, 30)

      range.popFront();
      writeln(range.front); // Date(2000, 2, 29)

      range.popFront();
      writeln(range.front); // Date(2001, 2, 28)

      range.popFront();
      writeln(range.front); // Date(2002, 2, 28)

      range.popFront();
      writeln(range.front); // Date(2003, 2, 28)

      range.popFront();
      writeln(range.front); // Date(2004, 2, 28)

      range.popFront();
      assert(range.empty);

nothrow TP delegate(in TP) `everyDuration`(TP, Direction dir = Direction.fwd, D)(D `duration`)
if (isTimePoint!TP && __traits(compiles, TP.init + `duration`) && (dir == Direction.fwd || dir == Direction.bwd));
  Range-generating function.
  Returns a delegate which returns the next time point which is the given `duration` later.
  Using this delegate allows iteration over successive time points which are apart by the given `duration` e.g. passing dur!"days"(3) to `everyDuration` would result in a delegate which could be used to iterate over a range of days which are each 3 days apart.
  Parameters:

  dir | The direction to iterate in. If passing the return value to fwdRange, use Direction.fwd. If passing it to bwdRange, use Direction.bwd.
  D `duration` | The `duration` which separates each successive time point in the range.

  Examples:

      import core.time : dur;
      import std.datetime.date : Date;

      auto interval = Interval!Date(Date(2010, 9, 2), Date(2010, 9, 27));
      auto func = everyDuration!Date(dur!"days"(8));
      auto range = interval.fwdRange(func);

      // Using PopFirst.yes would have made this Date(2010, 9, 10).
      writeln(range.front); // Date(2010, 9, 2)

      range.popFront();
      writeln(range.front); // Date(2010, 9, 10)

      range.popFront();
      writeln(range.front); // Date(2010, 9, 18)

      range.popFront();
      writeln(range.front); // Date(2010, 9, 26)

      range.popFront();
      assert(range.empty);

nothrow TP delegate(in TP) `everyDuration`(TP, Direction dir = Direction.fwd, D)(int `years`, int `months` = 0, AllowDayOverflow `allowOverflow` = AllowDayOverflow.yes, D `duration` = dur!"days"(0))
if (isTimePoint!TP && __traits(compiles, TP.init + `duration`) && __traits(compiles, TP.init.add!"`years`"(`years`)) && __traits(compiles, TP.init.add!"`months`"(`months`)) && (dir == Direction.fwd || dir == Direction.bwd));
  Range-generating function.
  Returns a delegate which returns the next time point which is the given number of `years`, month, and `duration` later.
  The difference between this version of `everyDuration` and the version which just takes a core.time.Duration is that this one also takes the number of `years` and `months` (along with an AllowDayOverflow to indicate whether adding `years` and `months` should allow the days to overflow).
  Note that if iterating forward, add!"`years`"() is called on the given time point, then add!"`months`"(), and finally the `duration` is added to it. However, if iterating backwards, the `duration` is added first, then add!"`months`"() is called, and finally add!"`years`"() is called. That way, going backwards generates close to the same time points that iterating forward does, but since adding `years` and `months` is not entirely reversible (due to possible day overflow, regardless of whether AllowDayOverflow.yes or AllowDayOverflow.no is used), it can't be guaranteed that iterating backwards will give the same time points as iterating forward would have (even assuming that the end of the range is a time point which would be returned by the delegate when iterating forward from begin).
  Parameters:

  dir | The direction to iterate in. If passing the return value to fwdRange, use Direction.fwd. If passing it to bwdRange, use Direction.bwd.
  int `years` | The number of `years` to add to the time point passed to the delegate.
  int `months` | The number of `months` to add to the time point passed to the delegate.
  AllowDayOverflow `allowOverflow` | Whether the days should be allowed to overflow on begin and end, causing their month to increment.
  D `duration` | The `duration` to add to the time point passed to the delegate.

  Examples:

      import core.time : dur;
      import std.datetime.date : AllowDayOverflow, Date;

      auto interval = Interval!Date(Date(2010, 9, 2), Date(2025, 9, 27));
      auto func = everyDuration!Date(4, 1, AllowDayOverflow.yes, dur!"days"(2));
      auto range = interval.fwdRange(func);

      // Using PopFirst.yes would have made this Date(2014, 10, 12).
      writeln(range.front); // Date(2010, 9, 2)

      range.popFront();
      writeln(range.front); // Date(2014, 10, 4)

      range.popFront();
      writeln(range.front); // Date(2018, 11, 6)

      range.popFront();
      writeln(range.front); // Date(2022, 12, 8)

      range.popFront();
      assert(range.empty);

struct `IntervalRange`(TP, Direction dir) if (isTimePoint!TP && dir != Direction.both);
  A range over an Interval.
  `IntervalRange` is only ever constructed by Interval. However, when it is constructed, it is given a function, func, which is used to generate the time points which are iterated over. func takes a time point and returns a time point of the same type. For instance, to iterate over all of the days in the interval Interval!Date, pass a function to Interval's fwdRange where that function took a std.datetime.date.Date and returned a std.datetime.date.Date which was one day later. That function would then be used by `IntervalRange`'s popFront to iterate over the std.datetime.date.Dates in the interval.
  If dir == Direction.fwd, then a range iterates forward in time, whereas if dir == Direction.bwd, then it iterates backwards in time. So, if dir == Direction.fwd then front == interval.begin, whereas if dir == Direction.bwd then front == interval.end. func must generate a time point going in the proper direction of iteration, or a std.datetime.date.DateTimeException will be thrown. So, to iterate forward in time, the time point that func generates must be later in time than the one passed to it. If it's either identical or earlier in time, then a std.datetime.date.DateTimeException will be thrown. To iterate backwards, then the generated time point must be before the time point which was passed in.
  If the generated time point is ever passed the edge of the range in the proper direction, then the edge of that range will be used instead. So, if iterating forward, and the generated time point is past the interval's end, then front becomes end. If iterating backwards, and the generated time point is before begin, then front becomes begin. In either case, the range would then be empty.
  Also note that while normally the begin of an interval is included in it and its end is excluded from it, if dir == Direction.bwd, then begin is treated as excluded and end is treated as included. This allows for the same behavior in both directions. This works because none of Interval's functions which care about whether begin or end is included or excluded are ever called by `IntervalRange`. interval returns a normal interval, regardless of whether dir == Direction.fwd or if dir == Direction.bwd, so any Interval functions which are called on it which care about whether begin or end are included or excluded will treat begin as included and end as excluded.
  pure nothrow ref IntervalRange `opAssign`(ref IntervalRange `rhs`);
  pure nothrow ref IntervalRange `opAssign`(IntervalRange `rhs`);
    Parameters:

    IntervalRange `rhs` | The IntervalRange to assign to this one.

  const pure nothrow @property bool `empty`();
    Whether this IntervalRange is `empty`.
  const pure @property TP `front`();
    The first time point in the range.
    Throws:
    std.datetime.date.DateTimeException if the range is empty.
  void `popFront`();
    Pops front from the range, using func to generate the next time point in the range. If the generated time point is beyond the edge of the range, then front is set to that edge, and the range is then empty. So, if iterating forwards, and the generated time point is greater than the interval's end, then front is set to end. If iterating backwards, and the generated time point is less than the interval's begin, then front is set to begin.
    Throws:
    std.datetime.date.DateTimeException if the range is empty or if the generated time point is in the wrong direction (i.e. if iterating forward and the generated time point is before front, or if iterating backwards and the generated time point is after front).
  pure nothrow @property IntervalRange `save`();
    Returns a copy of this.
  const pure nothrow @property Interval!TP `interval`();
    The `interval` that this IntervalRange currently covers.
  pure nothrow @property TP delegate(in TP) `func`();
    The function used to generate the next time point in the range.
  const pure nothrow @property Direction `direction`();
    The Direction that this range iterates in.
struct `PosInfIntervalRange`(TP) if (isTimePoint!TP);
  A range over a PosInfInterval. It is an infinite range.
  `PosInfIntervalRange` is only ever constructed by PosInfInterval. However, when it is constructed, it is given a function, func, which is used to generate the time points which are iterated over. func takes a time point and returns a time point of the same type. For instance, to iterate over all of the days in the interval PosInfInterval!Date, pass a function to PosInfInterval's fwdRange where that function took a std.datetime.date.Date and returned a std.datetime.date.Date which was one day later. That function would then be used by `PosInfIntervalRange`'s popFront to iterate over the std.datetime.date.Dates in the interval - though obviously, since the range is infinite, use a function such as std.range.take with it rather than iterating over all of the dates.
  As the interval goes to positive infinity, the range is always iterated over forwards, never backwards. func must generate a time point going in the proper direction of iteration, or a std.datetime.date.DateTimeException will be thrown. So, the time points that func generates must be later in time than the one passed to it. If it's either identical or earlier in time, then a std.datetime.date.DateTimeException will be thrown.
  pure nothrow ref PosInfIntervalRange `opAssign`(ref PosInfIntervalRange `rhs`);
  pure nothrow ref PosInfIntervalRange `opAssign`(PosInfIntervalRange `rhs`);
    Parameters:

    PosInfIntervalRange `rhs` | The PosInfIntervalRange to assign to this one.

  enum bool `empty`;
    This is an infinite range, so it is never `empty`.
  const pure nothrow @property TP `front`();
    The first time point in the range.
  void `popFront`();
    Pops front from the range, using func to generate the next time point in the range.
    Throws:
    std.datetime.date.DateTimeException if the generated time point is less than front.
  pure nothrow @property PosInfIntervalRange `save`();
    Returns a copy of this.
  const pure nothrow @property PosInfInterval!TP `interval`();
    The `interval` that this range currently covers.
  pure nothrow @property TP delegate(in TP) `func`();
    The function used to generate the next time point in the range.
struct `NegInfIntervalRange`(TP) if (isTimePoint!TP);
  A range over a NegInfInterval. It is an infinite range.
  `NegInfIntervalRange` is only ever constructed by NegInfInterval. However, when it is constructed, it is given a function, func, which is used to generate the time points which are iterated over. func takes a time point and returns a time point of the same type. For instance, to iterate over all of the days in the interval NegInfInterval!Date, pass a function to NegInfInterval's bwdRange where that function took a std.datetime.date.Date and returned a std.datetime.date.Date which was one day earlier. That function would then be used by `NegInfIntervalRange`'s popFront to iterate over the std.datetime.date.Dates in the interval - though obviously, since the range is infinite, use a function such as std.range.take with it rather than iterating over all of the dates.
  As the interval goes to negative infinity, the range is always iterated over backwards, never forwards. func must generate a time point going in the proper direction of iteration, or a std.datetime.date.DateTimeException will be thrown. So, the time points that func generates must be earlier in time than the one passed to it. If it's either identical or later in time, then a std.datetime.date.DateTimeException will be thrown.
  Also note that while normally the end of an interval is excluded from it, `NegInfIntervalRange` treats it as if it were included. This allows for the same behavior as with PosInfIntervalRange. This works because none of NegInfInterval's functions which care about whether end is included or excluded are ever called by `NegInfIntervalRange`. interval returns a normal interval, so any NegInfInterval functions which are called on it which care about whether end is included or excluded will treat end as excluded.
  pure nothrow ref NegInfIntervalRange `opAssign`(ref NegInfIntervalRange `rhs`);
  pure nothrow ref NegInfIntervalRange `opAssign`(NegInfIntervalRange `rhs`);
    Parameters:

    NegInfIntervalRange `rhs` | The NegInfIntervalRange to assign to this one.

  enum bool `empty`;
    This is an infinite range, so it is never `empty`.
  const pure nothrow @property TP `front`();
    The first time point in the range.
  void `popFront`();
    Pops front from the range, using func to generate the next time point in the range.
    Throws:
    std.datetime.date.DateTimeException if the generated time point is greater than front.
  pure nothrow @property NegInfIntervalRange `save`();
    Returns a copy of this.
  const pure nothrow @property NegInfInterval!TP `interval`();
    The `interval` that this range currently covers.
  pure nothrow @property TP delegate(in TP) `func`();
    The function used to generate the next time point in the range.

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