IIRC, we didn't have exact registration dates for some or all of the people who signed up on the old classicgaming.com ASP Forum, so when MB set up the SMF board he set the unknown join dates to the date and time of the users' first posts.
It was slightly more complicated/accurate than that. I set it to the date and time of the user's first post or if they didn't post right away, the next new user who posted's first post date and time.
