From what I recall, back when lucasforums merged (it used to be separate sites for jedi knight stuff, monkey island, etc) all the forum users got merged and since some of the seperate forums were older than others there were glitches in the join date for specific users. I believe since it was unknown, it basically defaulted to Unix Epoch, i.e. Jan 1 1970.
zX's join date is simply a joke that he (i think) edited into his profile
any ye olde admins who remember more than I do, or remember it more correctly, feel free to chime in and destroy my faith in my memory.