There is No Such Thing as Absolute Time
According to Einstein’s special theory of relativity, there is no such thing as absolute time. What this means is that there is no universal ‘clock’ ticking the time - every frame of reference has its own perception of time. Thus, there’s no such thing as an absolute present - or future, or past.
All time on Earth is relatively the same for all reference frames because all humans are not moving very fast - and nowhere near light speed. However, if we were moving at drastically different speeds, we would find that time ticks more slowly for the person moving more quickly - we would age at different rates! 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.