OK, I admit my English is not as good as it should be (and that my knowledge of Einsteins theories lacks a little, I must admit), so my last reply may have come out wrong.
But apart from that, it has been proved that time moves slower on moving objects (this has been done scientificly by placing an atomic clock on a Boeing 747, while another was placed on the ground). By comparing the two clocks after the flight, it showed that time had moved slower on the plane.
The theory of going back in time, goes something like this (once again, excuse my poor english):
If you could open up a wormehole, and place one end on Earth and the other at a spaceship going close to the speed of light, you could go through the wormhole from Earth to the spaceship, a place that was back in time compared to time on Earth. Don't hold me to this, but I'm sure that is how it was explained. You could of cours not go further back than the existens of the wormhole.
Anyway, timetraveling is mathematicly pheseable, but will remain a theory until proven wrong.
Hope that came out right