[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.