Alright, the best I can tell you is that someone could of hacked into your database and did that. If you have access to phpmyadmin, you can do it from there I believe. For the moment, I don't know what to edit but someone else does I believe. I'm sure it's slightly simple.
Actually, you're far from it.
December 31, 1969 at 23:59:59 is the date the UNIX epoch starts from. This is user #1, which is never used (as all accounts start from user #2. Because this account is never used, there is no registered date and therefore uses the default date the UNIX calendar was started.