Time is the continuous change of the future into the present. We split up time using measurements like hours, days, and years. Whenever I think about time, I think about traveling through it. Traveling backwards through time, I think, would be impossible. Going back in time would result in negative time, and in order to get into negative time, you would have to go past zero time. Since nothing would move or happen if there was no time to progress it, time would stay stopped. Time could possibly be sped up, but we wouldn't be able to comprehend this increased speed, because our methods of measuring it would would be moving with that time.
I don't really have any backing for this theory though.