I’ve been using MooTools MochaUI pretty extensively in building an intranet / administrative portal for a school. Unfortunately I have been unable to find a calendar that does not conflict with mocha on some level. We’ll finally a user on the mocha ui google group posted this calendar:
http://nsftools.com/tips/DatePickerTest.htm
It works great and can be made to look just as good as any of the mootools calendars through some simple CSS modifications.