When an appointment is on the pin board it is not connected to a clinic. Because it is not connected to a clinic, there is no time zone reference to associate with the edit in the Appointment History. In this case edits use UTC, which depending on your timezone and time of year (day light savings) the time will be off from the actual user date/time change. In some cases the Appointment History will show a date/time in the future from when the actual user action took place. Assign the correct UTC offset to all appointments.