A UnaryFunction that maps a number to the corresponding calendar
Year. For example, (YearFn 1912) returns the Class containing just one instance,
the year of 1912. As might be expected, positive integers return years in the Common Era,
while negative integers return years in B.C.E. Note that this function returns a Class
as a value. The reason for this is that the related functions, viz. MonthFn, DayFn,
HourFn, MinuteFn, and SecondFn, are used to generate both specific TimeIntervals
and recurrent intervals, and the only way to do this is to make the domains and ranges of
these functions classes rather than individuals.
