Atomic clocks are flown around the world, in different directions, and returned to a common location, and compared with a clock that remains behind.

The clocks are compared, and it is found that the clocks have actually experienced different durations of time, which can be calculated using time dilation given by Einstein’s relativity.

This direct experimental conformation of relativistic time dilation was first performed over thirty years ago. It is not an assumption. It is not a doppler shift. It is an observation of clocks measuring a real difference in the rates at which time passes in different frames.

Cheers — Chris