Atomic alarm clocks are alarm clocks that automatically synchronize themselves to the official U.S. master clock by receiving WWVB radio transmissions. These transmissions contain time information from the U.S. atomic clock source, operated by the U.S. Commerce Department near Boulder, Colorado.  Of course in order to work correctly, the clock must be in a location where the radio transmissions reach the clock.  Please see the additional information at the bottom of the page.

These atomic clocks can receive time broadcasts from the U.S. atomic clock transmitter in Colorado and synchronize itself to the U.S. atomic clock every night. As long as the signal is strong where the clock is located, the clock will maintain accuracy to a fraction of a second and reset itself for daylight saving time (unless you choose to disable the DST function). However, if the clock is located where the signal is poor, due to metal in the construction of your building blocking the signal or due to electrical interference from nearby electrical appliances or computers, then the clock may not always receive an adequate signal. Under these conditions, the clock may still synchronize occasionally when weather conditions are ideal (as weather also affects signal strength), or may need to be set manually.