An atomic clock is usually set by adjusting the desired time zone then allowing it to synchronize with the signal from radio station WWVB, which broadcasts the time of the atomically controlled clock. WWVB, established by the National Institute of Standards and Technology, covers most of the continental United States.
Atomic clocks that are sold to the public are actually quartz movement clocks that synchronize their time at regular intervals with the official atomic time broadcast by WWVB. A true atomic clock uses microwave signals to measure energy changes within caesium atoms that are cooled to near absolute zero. The NIST-F1 is one of the primary atomic clocks used by the United States for time distribution services.