Atomic clocks were used in Hafele and Keatings experiment which supposedly helped to prove time dilation.
Time Dilation Proof - Hafele and Keating
How can we be sure other forces didn't act upon the clocks as a result of their being at different speeds and elevations? Perhaps the elapsed time may have been the same for all and only appeared different as a result of the clock's atoms behaving differently in different environments.
How might the presence of dark matter (presumably all over the place) impacted the clocks if say the dark matter is stationary relative to the earth's rotation?
What experiments have been done to test the constancy of clocks at different speeds and elevations?