DateTimeException if the given interval
is empty.
Example:
assert(PosInfInterval!Date(Date(1996, 1, 2)).isAdjacent(
Interval!Date(Date(1989, 3, 1), Date(1996, 1, 2))));
assert(!PosInfInterval!Date(Date(1999, 1, 12)).isAdjacent(
Interval!Date(Date(1999, 1, 12), Date(2011, 9, 17))));