My understanding is that if X moves relative to a 'stationary' observer Y (say in direction +x), at close to the speed of light, time will pass 'more slowly' for X. That is to say, when X returns to Y, Y is older than X. But, isn't it just as accurate to say that Y is moving relative to X (in direction -x)? That being the case, Y would experience time dilation, and when X & Y meet up again, X should be older than Y. What gives?