In another comment at Language Hat on my post on The Ides of March, John Emerson suggests the development of a universal calendar converter as a generalization of the Kalendae program that I mentioned. That would be a neat project, but I suspect that it would be harder than he thinks.
The problem is that in some traditions there is apparently a tremendous amount of variation in detail. In India, for example, there are evidently many different calendars still in use, for which see the very interesting senior thesis [PDF file] by Leow Choon-Lian. She has an associated Mathematica package. Her advisor, Helmer Aslaksen, has a lot of information about various calendars here
The Muslim calendar appears from a distance to be universal within Islam, but it seems that there is considerable local variation in how the time of first visibility of the lunar crescent is determined, which can create disparities of as many as two days in when one month changes to the next. Robert Harry van Gent has an Islamic calendar converter together with information on the problems of conversion as well as a discussion (in Dutch) of Islamic chronology. It's an interesting subject, and probably would make a good open source project, but I fear not an easy one.
[A major collection of calendrical information is the book Calendrical Calculations by Edward M. Reingold and Nachum Dershowitz and the associated web site.]
Posted by Bill Poser at March 17, 2007 01:09 AM