Well, birth-year. [Not even going to PRETEND that we’ll ever be blessed with an actual DATE to celebrate XD]
It’s always puzzled me that John’s line in KKBB, “Rear of the year 5094.” in regards to Jack was never considered much more than a passing joke (“Still lookin’ good!”). I mean. Those two clearly knew each other well, beyond Time Agency missions surely?? [I have a whole seperate post about my headcanons on their relationship but that’s not the point here.] And before you try to tell me things like “maybe he snug off during a Time Agency mission to attend that contest (on a dare or smth??)”, or “maybe [assuming he and John really spend more time together] it happened during on of their cons”, or anything else that’d suggest it was just a random point in time that he visited - keep in mind that he was actually born in the 51st century. That’s canon and we know it. SO HE PROBABLY WOULDN’T TRAVEL TO THE END OF THE 51ST CENTURY (THE CENTURY HE LIVES IN (/ IS SUPPOSED TO LIVE IN)), IF ONLY IN FEAR OF SPOILERING HIMSELF OR WORSE, RUNNING INTO HIMSELF / CREATING A PARADOX (and I’m pretty damn sure at least the Time Agency had rules against that).
The only remaining explanation? It has to be a contest he attended within his own, original timeline. And if we can agree that Jack was likely in his early 30s when he met the Doctor, and that that was at least a few years after he’d last seen John (considering that otherwise, ‘still’ wouldn’t be such a compliment / they wouldn’t make so much fun out of each other looking ‘old’ if Jack didn’t actually look older than John remembered him, and since he hasn’t aged from that day on…) - he’d be born in the late 5060s, roundabout.
[personal favorite headcanon (and quite possibly, what Jack’d tell everyone if he could get away with it, be it true or not): 5069]