/// <summary>
/// Returns the union between the given interval and this interval, as long as they're overlapping or contiguous.
/// </summary>
/// <param name="interval">The specified interval from which to generate the union interval.</param>
/// <returns>
/// A <see cref="DateInterval"/> corresponding to the union between the given interval and the current
/// instance, in the case the intervals overlap or are contiguous; a null reference otherwise.
/// </returns>
/// <exception cref="ArgumentException"><paramref name="interval" /> uses a different calendar to this date interval.</exception>
public DateInterval?Union(DateInterval interval)
{
ValidateInterval(interval);
var start = LocalDate.Min(Start, interval.Start);
var end = LocalDate.Max(End, interval.End);
// Check whether the length of the interval we *would* construct is greater
// than the sum of the lengths - if it is, there's a day in that candidate union
// that isn't in either interval. Note the absence of "+ 1" and the use of >=
// - it's equivalent to Period.DaysBetween(...) + 1 > Length + interval.Length,
// but with fewer operations.
return(Period.DaysBetween(start, end) >= Length + interval.Length
? null
: new DateInterval(start, end));
}