UltimateBB software, like all software, evolves. Some time in 1999 the UltimateBB folks (now InfoPop - which has absorbed WWWThreads, which I almost changed to last year) changed their software to record the date someone registered - which it didn't previously. It wasn't, as I remember, in reaction to Y2K, but rather the 'feature' was added because of user (customer) demand.
UltimateBB did not use a database structure at all until they absorbed WWWThreads. It was all perl scripts which 'distilled' posts into threads which were streight HTML. This software (vBulletin) is the first forums software I have which uses a real database - not to mention php (which is somewhat like XML).
As for the vBulletin folks who wrote several importers (obviously to get UBB and other web bulletin board customers to change over 'easily'), I never did actually look at the code to see what the import script was doing. All I really know is where it could not find a 'joined' date for a member it inserted a 1970 date. But I don't see it as related to Y2K or database structure.
That said, if you really want to know who has been coming here the longest, you have to look at http://16949.com/level2/old_forum_archive.html
Unfortunately, the vBulletin people have not seen fit to write an importer for threads from WWWBoard (or whatever it was that I was using back in late 1996 through June 1998). I would really have liked that 'cause there are a lot of good old 'historical' threads there.
Some folks have been visiting the Cove Forums since mid-1996! Most - probably 99% - come, get what they need and move on never to return, however. In fact, I would bet most people who register now are doing so because they can see an attachment they want but can't download it unless they join. So it goes...