large to small could go 2021-Jan-02nd.
Suppose want it in U.S. style, 01/02/'21, where 01/01 = January 1st = day#1 of the year (2021).
01|31
02|28,29
03|31
04|30
05|31
06|30
07|31
08|31
09|30
10|31
11|30
12|31
02 # days, February has 28 on non leap-years; starting starting from 2000: {'01,'02,'03}+4y. 29 on leap day, every fourth year; starting from 2000: {'00}+4y
good so far? How to proceed? Any elegant formula?