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)));