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))); --------------------