I spent a while studying calendars some years ago; one of my more long-lasting obsessions. I had gotten a copy of Calendrical Calculations by Reingold and Dershowitz, in which they analyze and describe many calendars, both modern and ancient, and also discuss how to convert dates among them all. I thought that was cool, and as I had been just learning Java at the time, decided to make a Java applet to implement many of their conversions. (There's actually a much better one at their site). That has been a popular feature of my website for a long time, and so of course it is still available, as you can see in the menu. It hasn't yet been reconformed to my New and Improved website design, though.
I got on a calendar mailing list, in which people discussed all kinds of calendars and proposals for new alternate ones. Everyone keeps trying to solve the problem of 365(+change) days not being divisible into a whole number of weeks or twelve equal-length months. Many of the solutions are the same: have twelve 30-day months and five monthless days (works for the French Revolutionary calendar). Or 52 seven-day weeks plus one weekless day. The mailing list was active and interesting—and strangely heated for what one might think was a dry topic! (I can't seem to find the archives online yet). You might check out the Calendar Zone for some interesting alternate calendars.
Less rambling on this page than most, probably because I haven't been into calendars in a while, but it gets high placement in the menu because of the popularity of the conversion applet.