_DateToSSN Fuction return sequential serial number that represent a particular Date, and _SSNToDate Fuction return Date from sequential serial number (generated from DATE fuction on EXCEL or _DateToSSN Fuction, example 39637 = 7/8/2008), as already look at the function and too simple and uses few line of code, so that it is very fast (_DateToSSN processes about 55000 per second & _SSNToDate processes about 20000 per second) Global $DateSSN[27] = [0, 31, 29, 31, 30, 31, 30, 31, 31, 30, 31