The purpose of this book is to present in a unified, completely algorithmic form, a description of thirteen calendars and how they relate to one another: the present civil calendar (Gregorian), the recent ISO commercial calendar, the old civil calendar (Julian), the Coptic and (virtually identical) Ethiopic calendars, the Islamic (Moslem) calendar, the Baha'i, the Hebrew (Jewish) calendar, the Mayan calendars, the French Revolutionary calendar, the Chinese calendar, and both the old (mean) and new (true) Hindu (Indian) calendars. Easy conversion among these calendars is a by-product of the approach, as is the determination of secular and religious holidays. Calendrical Calculations makes accurate calendrical algorithms readily available for computer use. This volume will be a valuable resource for working programmers, as well as a fount of useful algorithmic tools for computer scientists.