I was looking for Java classes that support alternative calendar systems when I came across a book that looks like a goldmine for anyone doing this kind of programming: Calendrical Calculations by Edward Reingold and Nachum Dershowitz.
The Millenium Edition of the book includes chapters on the following calendars: Ba'hai, Balinese Pakuwon, Chinese, Coptic, Ecclesiastical, Ethiopic, French Revolutionary, Gregorian, Hebrew, Islamic, ISO, Julian, Mayan, Modern Hindu, Old Hindu, and Persian. It also includes a section with calendar implementations using Common Lisp.
You can read the book's preface, table of contents and a sample chapter on Reingold's web site. He also offers Calendrica, a Java applet that converts between different calendar systems.
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