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