Hi Lars and welcome to the forum!
Good question. Filling in events as much as possible on your calendar will ensure they are synced to The Events Calendar with as much information as possible. However, the iCal (.ics) format itself is not completely flexible to the point where it would be able to fill in all of the options that The Events Calendar makes available for an event in WordPress.
Also i tested with some all day event, and they were imported in the the calendar with the default dates ‘1970-01-01’
Ack, sorry about that. We are aware of this issue and have a ticket in our system to address it. In the meantime, if you replace the content of this file:
plugins/the-events-calendar/src/Tribe/iCal.php
…with this:
https://gist.github.com/jesseeproductions/d3b3b650ef36c9998635
That has fixed the issue in our test as well as with other customers. Does that work for you?
Cheers!
Geoff