Kind of unhandy: In WM6 try to enter an appointment from July 5th, 16:00 until July 6th, 4:00 (yes, a.m.). You can't do this, probably because you're not allowed to enter an end-time that is earlier than the start-time. What the programmer forgot, is that this rule should also depend on the start- and end-DATE.
The result is that if I have an appointment from 16:00, I need to stay at least until 16:00 the following day. Not all hosts like that :-)