YMD2DN
Convert from year and day number of year to month and day of month.
YMD2DN,yr,m,d,dy
yr = 4 digit year (like 1988), integer scalar m = month number (1-12, e.g. 11 = Nov) d = day of month (like 5).
dy = day number in year (like 310)
Adapted from ydn2md, Ole Streicher, 2015