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Time Realization and Distribution

The Time Realization and Distribution group distributes standard time and frequency signals generated by the Coordinated Universal Time scale, UTC(NIST), maintained at the NIST laboratories in Boulder, Colorado. UTC(NIST) is the U. S. national standard for time-of-day, time interval, and frequency.

UTC(NIST) is freely distributed to many millions of users through radio, Internet, and telephone links. The services that distribute UTC(NIST) include shortwave radio stations WWV and WWVH, low frequency radio station WWVB, the Internet Time Service (ITS), the Automated Computer Time Service (ACTS), the telephone time service, and the web clock ( These services synchronize hundreds of millions of clock every day.

UTC(NIST) is also distributed through remote calibration services to paying customers who need the highest levels of accuracy. The demanding measurement needs of these customers are met through the Frequency Measurement and Analysis Service (FMAS) and the Time Measurement and Analysis Service (TMAS).


Beginning November 15, 2021, WWV and WWVH will be broadcasting a test signal on minute 8 of each hour on WWV, and minute 48 on WWVH. This signal has been created to assist in ionospheric research, and is a joint effort of the Ham Radio Citizen Science Investigation (HamSCI) and NIST. The signal consists of various tones, chirps, and Gaussian noise bursts. The signal may be modified occasionally. For more information on HamSCI and the WWV/WWVH project, click here.

NBS/NIST Radio Stations: The Story of an Old Timer


This series of webpages will lead you on a journey on the history of the NBS/NIST radio stations. You can watch a video as well.

News and Updates

Projects and Programs


A Resilient Architecture for the Realization and Distribution of Coordinated Universal Time to Critical Infrastructure Systems in the United States: Methodologies and Recommendations from the National Institute of Standards and Technology (NIST)

Jeffrey Sherman, Ladan Arissian, Roger Brown, Matthew J. Deutch, Elizabeth Donley, Vladislav Gerginov, Judah Levine, Glenn Nelson, Andrew Novick, Bijunath Patla, Tom Parker, Benjamin Stuhl, Jian Yao, William Yates, Michael A. Lombardi, Victor Zhang, Douglas Sutton
The Time and Frequency Division of the National Institute of Standards and Technology (NIST), an agency of the United States Department of Commerce (DOC), was

A Century of WWV: 100th Anniversary Commemoration

Glenn K. Nelson
WWV was established as a radio station on October 1, 1919 with the issuance of the call letters by the U.S. Department of Commerce. This paper will observe the


SIM Time Scale

To support time and frequency metrology throughout the Americas, the Sistema Interamericano de Metrologia (SIM) Time and Frequency Metrology Working Group (TFMWG) maintains the SIM time scale (SIMT), the first continuously maintained multi-national ensemble time scale that is generated and published in real time (updated every hour). SIMT complements the world's official time scale, Coordinated Universal Time (UTC), by providing real time support to operational timing and calibration systems in the SIM region. The stability of SIMT is superior to most SIM local time scales and SIMT also provides a good approximation of UTC timing accuracy (±15 ns).

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Standard Reference Instruments (SRI)

NIST is consolidating its provision of reference instrumentation and devices into a new Standard Reference Instrumentation (SRI) program.

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