std.datetime.date.DateTimeException if either interval is empty.
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)));
Returns an interval that covers from the earliest time point of two intervals up to (but not including) the latest time point of two intervals.