How do they know?
Apparently scientists have made a clock that loses or gains only one second every 15 billion years. I can't see how this can be tested, proven or anything else.
https://www.sciencealert.com/physicists-have-broken-the-record-for-the-most-accurate-clock-ever-built
https://www.sciencealert.com/physicists-have-broken-the-record-for-the-most-accurate-clock-ever-built