I think time travel in ones own time from is impossible. The possibility for paradox is too great. Now if parallel universes existed, the idea that every major decision causes another parallel universe, it might be possible to travel back into a different time line. There are always stories, not always legitimate ones, that might account for time travelers. But I will point out Edgar Allen Poe's butterfly theory that someone mentioned earlier. The simple act of killing a butterfly could alter time irreversibly.