[idiotsecant]

Time crystals aren't the only thing that has properties that change in an orderly, predictable fashion. Plain old uranium has the same property. The answer to both is the same - put them on a very fast rocket and do a lap around the galaxy and see how well they agree with you on how much time has elapsed.

[illiac786]

Is it not rather that time does not elapse everywhere at he same speed, simply put?

Based on this, of course, measuring it becomes difficult.

It's akin to measuring gravity. You can — for a specific point in space and time. (Time is even worse, since it is specific to a given _trajectory_ in space)

But I have no idea what Im talking about.

[ben_w]

It's identical to measuring gravity. In relativity, acceleration and gravity are indistinguishable, and it's acceleration which breaks the paradoxical symmetry that gives the name to the twins paradox.