Time travel. He probably took a TARDIS and went to the year one hundred trillion. We don't know what happened to his TARDIS after he turned himself into a human to hide out.
Since Philip Segal was in charge of the telemovie, not the new tv series, does his opinion count any more than any other fan's? Russell T Davies, who was in charge of the new series, and invented the Time War, went out of his way to avoid definitively answering whether Gallifrey was in the distant past, the present, or the far future -- or even saying definitively if the question really has a meaningful answer.
Anyway, if a Time War is anywhen, it's everywhen!
- ↑ The fact that he had a chameleon arch implies that he must have had a TARDIS. For that matter, the fact that he was able to run away from the TIme War in the first place implies that he had one.