I haven't been keeping up with the nctm-l mailing list recently (so many messages! and my own job is quite busy lately. I had hoped to write something clarifying Brouwer and intuitionism and how little the subject has to do with the "constructivism" discussed here, but by the time I get to it, the topic will be long forgotten.) -- so I apologize if this reference has been cited already.
Anyway, on the subject of calendars and mathematics, my friend Ilan Vardi had a chapter in his book Mathematica Recreations (later retitled something like "Computer recreations in mathematics" -- to appeal to an audience a bit broader than Mathematica users), published by Addison Wesley on "mixed radix systems" which in effect, calendars are. He includes (if I remember correctly) information on Indian, Mayan, Julian/Gregorian, Jewish, Islamic and French Revolutionary calendars (though he omits the Italian Fascist calendar -- does anyone know the structure of it? Did they just rename the months?) and puts them in an interesting mathematical framework.