Hello.
I have the same problem with "Invalid date".
There are no visible problems in the defs folder.
Restarts, including those with administrative rights, did not help.
Repair didn't help either.
About 10 restarts were made.
This problem seems to have occurred since version 20. But I didn't pay attention to this before, because the antivirus works without problems. Updates are installed without problems. But recently, this "Invalid date" has become a nuisance. I found this topic. So I'm not the only one with this problem.
Most likely, the problem is really in the Avast antivirus.
And that makes me very upset.

A screenshot is attached.