NPR:
America’s master clock is one of the most accurate clocks on the planet: an atomic clock that uses oscillations in the element cesium to count out 0.0000000000000001 second at a time. If the clock had been started 300 million years ago, before the age of dinosaurs began, it would still be keeping time — down to the second.
Try that with your fancy-schmancy Apple Watch.
Try strapping an atomic clock to your wrist
Wait.
Atomic clocks are too accurate and have to be adjusted every year to match the slowing orbit of Earth (only a fraction of a second). World time is set in Greenwich, London, where an Atomic Clock sits, by they consult with French Astronomers server year to work out the “correct” time. Thus GMT is adjusted every year.
If you set an atomic clock 300 Million years ago, it would be way of the mark by now (by at least a few minutes!).