Thank you Ricco for taking a look into this.
I understand what you are saying.
The timestamp values get saved as 0000-00-00 00:00:00 in DB because the column they get saved in has format Datetime.
But before it was format timestamp and still the 0000-00-00 00:00:00 got saved instead of the correct timestamp.
As a workaround I am now saving in Datetime format and convert in PHP to timestamp format like this:
$dateobject = new DateTime($cck->getValue('news_dat'));
$timestamp = $termDate->getTimestamp();
This way I can use the timestamp to output the name of the month with:
echo strftime("%B", $timestamp);
I need the name of the month and this was my original reason to save the date in timestamp format to the DB.
Although my workaround solves the problem for me, I will leave the thread as open because I still don't have an answer why timestamp format doesn't get saved correctly to the DB.