You only ever need one time traveler convention. Because, of course, future time travelers can find out about it, and then come back in time to attend. The trick is publicity. So I am hereby doing my part, secure in the knowledge that Preposterous Universe will last for millennia, at least in archive form.
(Okay, I have to admit that this is somewhat bogus. A real time machine, i.e. a closed timelike path through spacetime, would only ever be able to take you back to the moment when it was originally created, not into the pre-existing past. So the convention will only work if someone has already built a time machine. But who knows?)