Computes the approximate number of years between two points in time.
sare datetime structures or timestamps
DateTime structures or timestamps, then
YearsBetween(t, s) returns the (in general non-integral) approximate number of years between
s, irrespective of the order of
s, based on the assumption that one year is 365.25 days.
YearsBetween(date(1980, 05, 20), date(2000, 05, 20))