system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If every part of the universe has aged differently owing to time running differently for each part, why do we say the universe is 13.8 billion years old?

For some parts relative to us, only a billion years would have passed, for others maybe 20?
gpt
We're simply judging by earth years not relativistic years. Sure technically, certain parts have advanced further in "time" due to the effect of gravity. But according to us here on earth, looking out. There has been approximately 13.8 billion years since the big bang. (A year being the time it takes for the earth to complete 1 full cycle around the sun)