Welcome to DEVONthink. I hope your trial is successful in you making a decision one way or the other to use or not use DEVONthink.
There are lots of different date fields in the list of “meta data”. Some dates like Date Added and Date Modified are never blank as they have to tell the truth about those details when the file is entered into the DEVONthink database.
What date field are you having problems with? How is the date field created–can you tell. Some sort of smart rule or something?