Simple as long as your precision is at milliseconds and you don’t account for space travel. We can measure the difference in speed of time in a valley and a mo

Simple as long as your precision is at milliseconds and you don’t account for space travel. We can measure the difference in speed of time in a valley and a mountain (“just” take an atomic clock up a mountain and wait for a bit, bring it back to your lab where the other atomic clock is now out of sync)