Right click the folder, select compact. EMail with a 1969 date are generally a sign the storage file is corrupted compacting should clear that. The only question is always what is lost when the corruption is removed so backup any email in the folder you want to keep before you do the compact.

You might also want to be looking for what caused the corruption in the first place. This is often an antivirus product that is scanning the file while Thunderbird is trying to write to it. Any activity that could cause contention is suspect, but by far the most common on windows is antivirus. The second is cloud backups to those that moved the mail profile to location like documents where it is not treated the same as it is in it's default location.