This video first goes into the history of clocks (for the first ~8 minutes) but then finally explains the latest most accurate atomic clock which uses quantum entanglement of cesium atoms to make it so accurate that if it was running from the time of the big bang then it would still be accurate to within about one-tenth of the second!
@humy Yeah, saw that. My job back in Apollo days was tracking and timing. Tracking was for a transponder on the craft that allowed very accurate reading of how far away Apollo was from Earth but timing part was the atomic clocks (Hewlett Packard off the shelf) and at Goddard Space Flight center, the first hydrogen maser clock, a lot more accurate than the cesium beam HP ones used in the field.
Now of course they would only rough in the timing of the big boys now🙂