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.