Take a sneak peek at the new NIST.gov and let us know what you think!
(Please note: some content may not be complete on the beta site.).

View the beta site
NIST logo

NIST Time Scales

NIST has two major time scales. The core scale, AT1, is a weighted average of commercial clocks, dominated by active hydrogen-masers. The most-used output is UTC(NIST), a real-time realization of the international standard, Coordinated Universal Time.

  • AT1 produces a stable frequency from the twelve minute measurement cycle out to several months. AT1 is generated every twelve minutes as a weighted average of several commercial atomic frequency standards. Dominated by about six active hydrogen masers, there are a number of commercial cesium beam-tube standards in the scale.
  • UTC(NIST) is a real-time realization of the international standard for time, Coordinated Universal Time, a weighted post-processed time scale. The time standard, UTC, is agreed by international treaty to be generated as a weighted average of clocks around the world. As such, it only exists after the fact. Any real-time version must be a prediction of what the defined value will be when it is published in circular-T.

Time Scale Data and Bulletin Archive

Updated monthly 

NIST no longer sends the Time and Frequency Bulletin by mail.
However, the bulletin is produced monthly as a PDF document that can be viewed using the Adobe Acrobat Reader.
The bulletins are available back to January 1997.

Download a Bulletin

Bookmark and Share

Time Scale Display

The NIST Time Scale Data is updated monthly. Direct questions to Petrina Potts: ppotts@nist.gov