Time dilation

Time dilation is the measurable fact that time does not run at the same rate everywhere. According to Einstein's relativity, a clock ticks slower the faster it moves (special relativity) and faster the further it is from a source of gravity (general relativity).

For GNSS this is not theory but daily practice: the clocks in the satellites run about 38 microseconds per day differently from those on Earth because of both effects, and that offset is actively corrected.