In running the sample SAS code backwards ( do year=2200 to 1 by -1; ) I notice the MDY function does not report any leap years earlier than 1584. The years 1600, 1596, 1592, 1588 and 1584 are leap years apparently, but that's it, no leap years are reported earlier than that. I'm wondering why ? Is it related to Pope Gregory's Edict in 1582 ?