The trouble is, When Einstein talked about time travel it was never in the sense that one day we would be able to build a machine that would allow us to travel back and forth in time. What he said was that if a person were in a vessell travelling at or close to the speed of light time would be percieved very differently from that person's perspective as opposed to someone stationary. In essence time traverses more slowly the closer to the speed of light you get. This is called Time Dilation. It's one of those things that lots of people kind of know but very few trully understand, and there are all sorts of variables in that theory.
On the flip side the traditional theory of time travel also poses it's own problems, too many paradox's pop up. Ultimately though if you were able to time travel and alter hsitory, no one would know about it because things would change but no one would know because memory and past events would also change with it. In theory someone could alter time every hour of every day and none of us would be any the wiser.