Atomic clocks tick differently with different gravity. That's a fact. This implies either one of the following statements: (1) Gravity affects time itself OR (2) Gravity affects time measurement only
Relativity supports hypothesis 1, but I wonder: is there any experimental evidence against hypothesis 2? Or the two are equivalent?
Example: a clock on the Earth measures 1 minute while a clock in the space measures 2 minutes. Why do we say that "time is slower on Earth" instead of just saying "clocks tick slower on Earth"? How can we be experimentally sure that time-dilation is real, and not just an illusion due to the side-effects of gravity on instrumentation?