I once read that they have a radio reciever in them tuned to 5 MHz, or 10 MHz. In the U.S. The stations on those Frequencies are WWV, in Fort Collins,
Colorado and WWVH in Hawaii.
Those Stations are run by The National Institute of Standards and Technology (Changed from National Bureau of Standards)
To find more information on those stations, and others like it, you could probably do a google search.
Here is what wikipedia gave me for WWV.
WWV is the callsign of NIST's shortwave radio station located in Fort Collins, Colorado. WWV's main function is the continuous dissemination of
official U.S. Government time signals. The station broadcasts simultaneously on five distinct frequencies: 2.5 MHz, 5 MHz, 10 MHz, 15 MHz and 20 MHz.
These carrier frequencies, as well as the time signals, are derived from a set of atomic clocks located at the transmitter site, which themselves are
traceable to NIST's primary frequency standard in Boulder, Colorado using such techniques as GPS common-view observations. WWV is partnered with
radio station WWVH, located in Hawaii. Onsite with WWV in Fort Collins, Colorado is also radio station WWVB, which operates on the low frequency of 60
WWV is the oldest continuously-operating radio station in the United States, first going on the air from Washington, D.C. in May of 1920,
approximately six months before the launch of KDKA. The station was formerly located in Greenbelt, Maryland, on land which now is part of NASA's
Goddard Space Flight Center. WWV moved to its present location at Fort Collins on December 1, 1966, enabling better reception of its signal throughout
the continental United States.
WWV is one of a rare number of radio stations west of the Mississippi River with a call sign beginning with W. The W callsign stems from the
station's early locations in D.C. and Maryland—the callsign was maintained when the federal government moved the station to Colorado—as well as
the fact that WWV, being a government station, does not fall within the FCC's jurisdiction with respect to call signs.
Hope this helps!