I've also been having fun playing with Calendar Magic. This was be an ideal tool if I could define custom calendars. There are so many calendars supported by this free app, however, that I think one could simply select one of the more obscure real-world calendars and repurpose it as a fantasy calendar. It will provide dual calendar views, calendar comparisons, and many other calendar and date tools. If moon phases are important to you, it has that too. Again, it is limited to real world, Earth calendars and moon phases. But it is still fun to play around with.
http://www.stokepoges.plus.com/calendar.htm