I’m struggling to wrap my mind around these things too.
So your answer seems to indicate that scaling up (to the level of the entire universe) gets one closer to being able to identify some absolute time. But scaling smaller brings out the relativistic difficulties. I would have guessed exactly opposite of that --that over large scales (e.g. between our galaxy and some other quite distant galaxy) there would be the greater relativistic differences due to radically different motion reference frames; and that, say, within our galaxy there would be smaller discrepancies between proposed “universal” clocks. But if I understand your answer correctly, we have more difficulties than ever with our closest neighbors. Logically extending that further in, shouldn’t globally agreed-upon “time stamps” be near impossible in the even smaller context of our planet? I think I’m misunderstanding your answer. But thanks in advance for your patience …