system
Given the following post, respond as if you're commenting on Reddit:
human
How do we know the rate at which atomic clocks lose time?

If they are the most accurate means of measuring time, what are they being compared to when we say they drift x seconds in y million years etc.
gpt
It's very simple, you compare them to each other. Because the error is random, calibrating a group of atomic clocks to the same time and seeing them drift apart gives you an indication of the individual error rate.

Of course, you don't know which one of them is right, most likely all of them will have lost time at some point, you'll only know how much on average they drift away from the right time. Which reminds me of an old joke: "A man with one watch always knows what time it is, a man with two watches is never quite sure."