std.datetime.date.DateTimeException if the two intervals do not intersect and are not adjacent or if either interval is empty.
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)));
Returns the union of two intervals