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Well, under some specified conditions, not involving accelerations/decelerations it actually define it. When it comes to accelerations/decelerations you can still argue that the discrepancy you measure (from 'c') is a result of 'gravity', locally measured.
There are two sets of relativity, one being SR, the other involving gravity (GR). And using SR 'c' is a constant.
There are a sort of 'global definitions' in physics, as Lorentz transformations. Ways of knitting one frame of reference to another, what I measure to what you measure. They are expressions of a local logic applied on a universe we need to agree on. We can't agree though, not without introducing time dilations and length contractions, to get those different measurements to fit, unless we happen to be in a same frame of reference as we measure.. We've been in one for the longest time btw, it's called Earth. That's also why we found it so easy to agree on our observations. It's not until recently we've found out otherwise. But a 'time dilation' has nothing to do with your life span. It always stay the same. locally measured, as by your wristwatch. A 'time dilation' is a result of you measuring locally, using that wristwatch (and that measuring stick), relative some other celestial object (neutron star maybe?) for example. It doesn't change your clock, neither your meter stick. I better point out that it doesn't change your length either, Age does though =And all of this are general descriptions. Being in a same 'frame of reference' can also be seen as being scale dependent, depending on your type/choice of measurement, as shown by NIST experiments with atomic clocks.