A new world, I had trouble getting the Nano and ESP8266 working and still having a little RAM left, so I am trying an ESP32 NodeMCU.   I call it a Nuclear Clock (Atomic Clock was already used) it's timekeeping depends on the energy state changes of cesium atoms passing through an RF field.   The cesium atoms "resonate" at a frequency of 9,192,631,770 Cycles Per Second.   As of January 2013, the uncertainty has been reduced to about 3 x 10-16, which means it would neither gain nor lose a second in more than 100 million years!   I notice that they don't refer to is as Hertz, probably because these guys are physicists not EEs, and they   The Mother load of infor about the USA's clock

NIST-F1

This from "www.livescience.com".  
When exposed to certain frequencies of radiation, such as radio waves, the subatomic particles called electrons that orbit an atom's nucleus will "jump" back and forth between energy states. Clocks based on this jumping within atoms can therefore provide an extremely precise way to count seconds. Inside a cesium atomic clock, cesium atoms are funneled down a tube where they pass through radio waves . If this frequency is just right 9,192,631,770 cycles per second then the cesium atoms "resonate" and change their energy state. A detector at the end of the tube keeps track of the number of cesium atoms reaching it that have changed their energy states. The more finely tuned the radio wave frequency is to 9,192,631,770 cycles per second, the more cesium atoms reach the detector. The detector feeds information back into the radio wave generator. It synchronizes the frequency of the radio waves with the peak number of cesium atoms striking it. Other electronics in the atomic clock count this frequency. As with a single swing of the pendulum, a second is ticked off when the frequency count is met. In the United States, the top clocks are maintained by the National Institutes of Standards and Technology (NIST) in Boulder, Colo., and the United States Naval Observatory (USNO) in Washington, D.C. The NIST-F1 cesium atomic clock can produce a frequency so precise that its time error per day is about 0.03 nanoseconds, which means that the clock would lose one second in 100 million years. Super-accurate timekeeping is integral to many elements of modern life, such as high-speed electronic communications, electrical grids and the Global Positioning System (GPS) and of course knowing when your favorite television show comes on.