The venerable CALNDR-L is an unmoderated email forum for the discussion of the social, historical and philosophical dimensions of Calendars and Time Reckoning. I discovered I had an interest in calendar systems in the years leading up to the millennium. When I joined in July 2002, the forum had already been running for six years, and today it celebrates its 10th anniversary. It's a fascinating list to follow and contains regular contributions from a number of world authorities on astronomical and calendrical matters.
A browsable version of the discussions can be viewed through Nabble.