DateTimeException if either interval is
empty.
Example: -------------------- assert(Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).span( Interval!Date(Date(1990, 7, 6), Date(1991, 1, 8))) == Interval!Date(Date(1990, 7 , 6), Date(2012, 3, 1)));
assert(Interval!Date(Date(1996, 1, 2), Date(2012, 3, 1)).span( Interval!Date(Date(2012, 3, 1), Date(2013, 5, 7))) == Interval!Date(Date(1996, 1 , 2), Date(2013, 5, 7))); --------------------