This is something I found fascinating and thought some of our users may find this interesting as well. Most of the clocks in my life are somehow synced with an atomic clock somewhere. Granted, this consists of my phone and computer but who needs a clock outside of these devices? This includes our own web site: www.worldtimeserver.com .
Atomic Clocks work by measuring what is essentially the vibration of excited atoms. Caesium is used in most cases but there are alternatives. Microwaves are used to excite the atoms to a specific frequency or level of vibration. The Atomic Clock checks the various caesium atoms for this vibration and when it finds them it knows things are vibrating at the right level and uses that frequency to increase time forward.
This is essentially the same thing as a pendulum clock. But microwaves are used to send the atomic pendulum moving and the pendulum is constantly checked for moving the exact right distance. Yes, the process is of course a lot more technical than this but I always find things a little easier to understand if I can picture a common equivalent.