Suppose that a man was in fact...

Not really, which is why i came to SP. i don't know math to this level.

Well, a light year is 9.4605284 × 10^15 meters and is best explained as the length light travels in one year. So 20 light years would be 20 x 9.4605284 x 10^15 or 1.89210568 x 10^17 meters. This is translated to 10000 or so billion meters(near to a trillion). So to answer your question, if the aliens knew how to travel at light speed and can travel that speed all the way back to say their home planet 20 light years away, it would be the same as 20 human years. So technically if they went back to their home world, stopped for a rest stop and came back, then 40 years would have gone by. It all depends on the distance you are going. Mind you there probably is a better way to travel the universe faster than the speed of light, we just haven't really gotten around to working out the technical way of achieving either space travel. Another good theory to add to the space travel idea is that the aliens may have a device that could simply teleport them or perhaps alter the matter around them to get them to a certain spot faster. Or maybe they even have a time machine. There are way too many variables here.