No clock keeps perfect time. For most of history clocks were set by the movement of the Sun and stars.
Since 1967 the world’s time has been set by atomic clocks.
Atomic clocks are accurate to 0.001 sec in 1000 years.
If a cesium atomic clock ran for six million years it would not gain or lose a second.
The world’s most accurate clock is the American NIST-7 atomic clock.
The atomic clock on the International Space Station is hundreds of times more accurate than clocks on Earth, because it is not affected by gravity.
Atomic clocks work because cesium atoms vibrate exactly 9,192,631,770 times a second.
Some scientists say that time is the fourth dimension – the other three are length, breadth and height. So time could theoretically run in any direction. Others say time only moves in one direction. Just as we cannot unburn a candle, we cannot turn back time.
Light takes millions of years to reach us from distant galaxies, so we see them not as they are but as they were millions of years ago. Light takes a little while to reach us even from nearby things.
Einstein’s theory of general relativity shows that time actually runs slower nearer strong gravitational fields such as stars. This does not mean that the clock is running slower but that time itself is running slower. Time also goes slower as speed increases.