Notification times are often wrong (using Google calendar)

* added new Google calendar and entered credentials
* events got synchronized

Now when viewing events the notification times on many events are
* off (e.g. '3 hours' instead of '4 hours' or '2 days instead of '1 hour') or
* completely wrong (e.g. only 1 notification with '10 minutes' instead of 2 notifications with '1 day' and '2 hours')

Note: I am mostly using several notifications for an event (e.g. '1 week', '1 day' and '1hour' or '1 day', '3 hours', '30 minutes').

Also, first I thought it was a time zone issue (i am in the 'Berlin' time zone) because many notification times were off by 1 hour but then I also saw that some events had totally wrong notifications...