About calendars -- A US Navy astronomical guide

This is a great non-religious resource to learn all about calendars.

clipped from aa.usno.navy.mil Introduction to Calendars Astronomical Applications Department A calendar is a system of organizing units of time for the purpose of reckoning time over extended periods. There are six principal calendars in current use. These are the Gregorian, Hebrew, Islamic, Indian, Chinese, and Julian Calendars. These calendars replicate astronomical cycles according to fixed rules. The data and information on our web site are limited to the astronomical issues for these calendars.