DateTimeException if either interval is
empty.
Example: -------------------- assert(Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).isAdjacent( Interval!Date(Date(1990, 7, 6), Date(1996, 1, 2))));
assert(Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).isAdjacent( Interval!Date(Date(2012, 3, 1), Date(2013, 9, 17))));
assert(!Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).isAdjacent( Interval!Date(Date(1989, 3, 1), Date(2012, 3, 1)))); --------------------