How to check the accuracy of a clock in the real world?

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For that particular clock model, buy another exact copy. If the 2 copies keep in sync, then they are accurate, since the probability of their random errors match is extremely low.

How about systematic errors?

— Me@2015-09-14 09:15:35 AM

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Compare that clock with another with a different model.

Even if the other model’s clock may be with lower accuracy, it can help you to find the systematic errors of your clock model.

— Me@2015-09-14 10:00:56 AM

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