Clock hypothesis
The clock hypothesis is an assumption in special relativity. It states that the rate of a clock does not depend on its acceleration but only on its instantaneous velocity. This is equivalent to stating that a clock moving along a path measures the proper time, defined by:
The clock hypothesis was implicitly (but not explicitly) included in Einstein's original 1905 formulation of special relativity. Since then, it has become a standard assumption and is usually included in the axioms of special relativity, especially in the light of experimental verification up to very high accelerations in particle accelerators.
...
Wikipedia