The oldest service is radio station WWV, near Fort Collins, Colo. It can be received all over the world and broadcasts at 2.5, 5, 10, 15 and 20 megahertz on the short wave band. A sister station, WWVH, broadcasts to the Pacific basin from Kauai, Hawaii, on everything but the 20 megahertz frequency. The broadcasts include standard frequencies and time intervals, the time (both voice and digital code), astronomical time corrections, and public service announcements such as marine weather, geophysical alerts and radio propagation information. The accuracy of these broadcasts, as received, is between one and 100 milliseconds, depending on the distance to the receiver. A telephone time service, which carries the WWV broadcast, is available by dialing (303) 499-7111 (a toll call outside the Denver Metro area). WWV has been broadcasting since 1923.

Another NIST radio station, WWVB, broadcasts a digital time code at the standard frequency of 60 kilohertz. Also located near Fort Collins, this low frequency station, which covers the continental U.S., has an accuracy for time comparison of between 0.1 and one millisecond. Its 60 kilohertz carrier frequency can be used for frequency calibrations with an accuracy of better than 1 part in 100 billion.

The Automated Computer Time Service (ACTS) provides millisecond accuracy for computer clocks and other digital systems using dial-up telephone lines. This service includes automatic compensation for telephone-line delay and advance notices of changes to and from daylight savings time as well as leap seconds. NIST's time signals also are available on the Internet. A visual display of the correct time can be seen at www.time.gov, a service provided jointly with the U.S. Naval Observatory.

NIST time can also be obtained via the Internet to set the internal clocks of computers automatically, using free software available at www.bldrdoc.gov/timefreq/service/nts.htm.