If we were moving at very different speeds, we would find that one of us aged quicker than the other. Similarly, if one of us was closer than the other to a major gravity well like the Earth, we would age slower than someone who wasn’t.
GPS satellites, of course, are both moving quickly and at significant distances from Earth. So their internal clocks show a different time to the receivers on the ground. A lot of computing power has to go into making your sat-nav work around the theory of special relativity.