Steps to reproduce my issue:
1. Be in a time zone different than GMT and create a new all day appointment spanning two or three days.
2. Wait for the appointment to sync to Outlook.
3. Ask a colleague who does not have Editor rights on your Outlook calendar (e.g. "Can view titles and locations" permission) to take a look at your calendar.
4. The all day event is now starting a little later and ending a little later.
5. Change to permission of your calendar for test colleague from 3. and let him look again at calendar.
6. Now the appointment should appear as all day event starting and ending at midnight.
How can I make sure that the appointment is always shown correctly?