Eli5 if time is relative and gets effected by gravity how could we possibly determine the age of our universe?

Question

Eli5 if time is relative and gets effected by gravity how could we possibly determine the age of our universe?

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Answer 1

Time is relative always involves frames of reference. One’s own clock always ticks normally; one might see a different clock ticking at a different rate depending on velocity and or gravity at that other clock.

We measure the age of the universe from the frame of reference of the cosmic microwave background . If we chose a different frame we would get a different value.