When you enter an appointment, it hijacks the date for today's date. So if you go to enter the appointment for May15th at 10:00 a.m. it will schedule it for today at 10:00 a.m. You then need to go back into the appointment and fix th date again.
A tech advisor was able to record this issue and replicate it a week ago when I was talking to them.
I ran a plug-in conflict test this week-end and even with all plug-ins deactivated the problem still seems to be happening. It is happening when you enter in an appointment manually via the back-end. If you enter from the calendar on the main site it seems to go in no problem.