Just to make sure we’re aligned:
- The Library is meant to be the place where you keep track of the media you own like DVD, Blu-ray, etc. The metadata tags are displayed in the Library.
- Adding something to your Library is not the same as marking it “watched”. Marking something Watched at “Unknown Date” means you’ve watched it but you don’t know or don’t care about the exact date. The Unknown Date is a group in your Watched list.
Now, are you trying to do one or the other or both?
If you are trying to do something else and I’m misunderstanding your point, please let me know and send me a screenshot, it will help 
Thanks!