Sorry for eventual factual errors, I am just hobbyist.
A spaceship flies with nearly speed of light away from earth and back. Time on board were going slower than on earth during flight, so back on earth the pilot is younger than his twin on earth, also the pilots clock show that less time got by for him than for his friends on earth.
So far so good, but how does time "know" that the spaceship moves away from earth and not the other way around? I mean motion is relative so it could be also seen that earth (or the whole universe) moves away from the spaceship? To distinguish those case an absolute universal coordinate system would be needed which I don't believe exist. What am I missing here?