Finally made it myself (mostly). Even though the site has run fine for years and thousands of content items, after the update I had to change date format! Until then there was "Y-m-d H:i:s", now I use "d.m.Y" (German format) .
If you don't have hours/minutes/seconds, the plugin stores the correct date, but uses the time of saving instead of default time. I've added some lines to repair that. If anyones interested in it, place a post.
Also I think there's some trouble with having live values like "today". This is going to be my next task.