I'll try to explain to the best of my knowledge: There was once an event with PT (a server change, perhaps?) — and this caused all the posts to be set to the default of the earliest date possible, which in this case would be 1/1/1970, despite that PT wasn't even around then. So all posts that were made before that are marked as such, even if they only happened in 2000 or 2002 or whatever. All posts made after that event are marked correctly — but you'll still see 1/1/1970 on posts, even though that's not when they were really made — it's just the default date that all posts were given during the event. New posts made after that were able to rewrite the default with the correct date, but all the posts made before that event lie dormant and marked 1/1/1970.