I forget the exact scientific formula behind it (it’s in most college intro physics books though, speed of light stuff, quantum mechanics stuff), BUT the faster you go the faster time travels for you. The caveat here is that in order to have a noticeable difference you need to be going VERY fast (like 1/4 or more the speed of light). I would assume they used atomic clocks for that reason, they can pick up tiny differences. ALSO, they know that concentrations of gravity slow time (no clue why though). They have done similar tests by placing clocks in a low subterranean basement and another at the top of a mountain, and found the one on the mountain (further from the gravity source) went faster by a tiny amount. This gravity concept is what lead to the Einstein time travel idea. THEORETICALLY we know how to time travel. You would create a black hole somewhere in space (which is a massive gravity pull), and you would create a wormhole. You would place one end of the wormhole at the black hole, and the other some great distance away in empty space (or simply an area with a much lower gravity pull). Time would travel slower at the black hole side (significantly so), so if you went into the wormhole at the other end and came out at the black hole side, you would have gone back in time. The only limits would be you could only go as far back as the black hole was made (that and you have to make a black hole, worm hole, space ship, etc) ....