In Buyer's Guides: Atomic Clocks
Atomic clocks receive a radio signal from the most accurate clock in the world, the cesium atomic clock, transmitted from the National Institute of Standards in Fort Collins, CO. The radio signal that covers the entire United States will keep your clock accurate to within one second over one million years. You'll never have to reset the clock, even for daylight-saving time!
The US Naval Observatory which operated 70 "atomic clocks" provided the following background information about these clocks: 'A cesium beam atomic clock uses the exact frequency emitted by atoms of the metallic element cesium. Today, cesium clocks measure frequency with an accuracy of one second in 1,400,000 years! It is the most accurate realization of a unit that mankind has yet achieved. A cesium clock operates by exposing cesium atoms to microwaves until they vibrate and counting the corresponding cycles as a measure of time.