Apologies if this is a dupe of 0001240, I can't quite tell. Likewise apologies if this has been fixed already, we're currently on 2.2.17a with no real way to test a more recent version.

In the Europe/Berlin time zone, daylight saving comes into effect on the last Sunday of March. If, like this year, this happens to be the 25th (which means 6 more March days following (next time this happens will be in 2029...)), then SOGo's idea of when the change happens seems off by a week for reasons that evade me.

The result is that any calendar entries set to take place in the week before the the 25th (March 18 - March 25) will have their start and end times offset by one hour. The problem is not visible in the Web frontend itself, but inside the database the timestamps are off by an hour. Synchronization with external clients (Thunderbird + Lightning in this case) reveals the issue.