Requires one argument (date, date time, int, or string of ints)
Minimum input is YYMMDD
Optional argument FORMAT_RETURN
If all the elements of the date and time are in the proper range for
the given year and month then the input is returned. Otherwise NULL
Additionally if the argument is numeric or a numeric string and
FORMAT_RETURN is the 2nd argument, then the 1st argument is returned
with standard MySQL formatting.
PS. The recent request for max_month_days() could easily be added if you
folks still want it.