Mark your past and future calendars, "Phantom Toll Booth" devotees. A time traveler convention, being billed as the first, last and only, will reportedly take place at MIT's East Campus Courtyard on Saturday, May 7.Technically, only one such convention is needed, the press release points out, as time travelers from all eras could meet at a specific place and time, and then make as many repeat visits as they wanted. "Write the details down on a piece of acid-free paper," implores the announcement, "and slip them into obscure books in academic libraries! Carve them into a clay tablet! A note: Time travel is a hard problem, and it may not be invented until long after MIT has faded into oblivion. Thus, we ask that you include the latitude/longitude information when you publicize the convention." We realize this is kind of last-minute notice, so if you're busy next weekend, not to worry. You can always attend in 4678.