Book review: Calendrical calculations, by Nachum Dershowitz and Edward M. Reingold

What a wizard wheeze! A set of computer programs to calculate any date in any calendar and convert between them. Praise, first, for Dershowitz and Reingold, computational calendricologists, who have devoted a large part of their joint lives to a task which anyone in their right mind will be glad someone else did. Calendrical Calculations is presented by its publishers as 'definitive', 'accurate', 'useful', 'easy' and 'a must', which coming from CUP immediately arouses interest. Its purpose is 'to present, in a unified, completely algorithmic form, a description of fourteen calendars and how they relate to one another'. The world's main calendars are all here: Christian (both Gregorian and Julian), Hebrew, Hindu (both old and modern), Islamic, modern Persian, Coptic, Mayan and Chinese. There are also three modern reformed calendars, all of them effectively defunct: the Baha'i calendar, the French revolutionary calendar, and the ISO (International Standards Organization) calendar, an excessively sensible Swedish invention. Brief explanations are given of each, and there are valuable overview chapters on calendars in general and on time and astronomy. The bulk of the book, however, is given over to an explication of the algorithms into which the calendars are translated, in a computer language called LISP. These are set out in an appendix. The book comes complete with a license (yes, you are allowed to use it) and an associated website, bristling with errata.