Calendar Studies
Peter Meyer's articles on the Gregorian and Julian calendars, the ISO date format, the Julian day number system, C functions for date conversion, the Maya calendar and the Goddess Lunar Calendar plus software for calendrical conversion.
The Perpetual Calendar
Toke Norby's helpful Tool to Postal Historians. Emphasizes dates of change from one calendar
reckoning to another - especially Julian to Gregorian - in various locations.