DateTimeException if the given interval
is empty
Example: -------------------- assert(!NegInfInterval!Date(Date(2012, 3, 1)).isBefore( Interval!Date(Date(1990, 7, 6), Date(2000, 8, 2))));
assert(!NegInfInterval!Date(Date(2012, 3, 1)).isBefore( Interval!Date(Date(1999, 1, 12), Date(2011, 9, 17))));
assert(NegInfInterval!Date(Date(2012, 3, 1)).isBefore( Interval!Date(Date(2022, 10, 19), Date(2027, 6, 3)))); --------------------