Per my comments in T308081, I agree with @Quiddity that after looking a user up on Special:CentralAuth, the dates for users (e.g. Special:CentralAuth/RobLa) should point to the registration date or perhaps the "first edit" date. As it stands, the date (when hooked up to SUL for a specific user database) is misleading for everyone who edited Wikipedia prior to the 2015 SUL finalization, and it takes a Wikimedia Foundation history nerd (or someone who was there at the time) to understand how to use the date that's shown. Given how long it's been since the 2015 SUL finalization, there really not a great reasons for showing the "SUL attachment" date so prevalently without showing the "first edit" date (which is much more important, and more likely to be the thing that the user is interested in).

Well, I found something in the MediaWiki API that looks closer (from this API call). What I see:

"registrationdate": "2001-10-22T06:37:02Z",

That's still not correct, but it's closer. It would seem that the field is in the user_registration field in the user table of each wiki. The registration date can't be right for enwiki, because this change has my fingerprints all over it. This change is credited to me, which seems plausible from the description ("Added Google directory link"), since that was back in the day that Google was using data from Mozilla's Open Directory Project to prioritize their search results. It's really too bad that Google fails to acknowledge the importance of DMOZ in their early history.