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 

• Notes on NIST Time Scale, Primary Standards and Services
• Leap Second and UT1 - UTC Information
• UTC-UTC (NIST)
• UTC (USNO) - UTC (NIST)

NIST no longer sends the Time and Frequency Bulletin by mail. However, the bulletin is produced monthly as a PDF document that can be downloaded here. The bulletins are available from January 1997 to the present but we only display back to 2013 here. If you need an older bulletin, please email kathryn.stephenson [at] nist.gov (subject: Time%20Scale%20Bulletin) (Katy Stephenson).

The files are viewable using the Adobe Acrobat Reader.