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Answer: NIST's 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.

How the time is kept

The group operates and continuously analyzes a large ensemble of commercial atomic clocks. The stability and robustness of UTC(NIST) is enhanced by averaging across the clock ensemble. Its accuracy is ensured by regular comparisons with primary frequency references, such as the cesium atomic fountain, and with other national metrology laboratories.

The Internet Time Service

• NIST operates several stratum-1 network time servers, which means their time is directly linked to UTC(NIST).
• The service responds to time requests from any Internet client in several formats including the DAYTIME, TIME, and NTP protocols.
• Requests in these formats generally do not support authentication, and no keys or passwords are needed.
• NIST also provides authenticated NTP messages using a symmetric-key algorithm, which ensures a message originated from a NIST time server and was not modified during transit.
• Most operating systems, such as Windows, MacOS, and Linux, have an option to automatically synchronize the system clock by sending queries to an NTP server.

Leap seconds

The name of a positive leap second is 23:59:60, but digital time formats such as NTP cannot represent that time. The NIST time servers add the extra leap second by stopping the clock for one second at 23:59:59.

Other services

NIST is now offering a network time service to deliver UT1 time.

Claims

• id: C1 claim: NIST operates several stratum-1 network time servers. source: S1 evidence: "NIST operates several stratum-1 network time servers"
• id: C2 claim: Other sources of time are not monitored or certified by NIST. source: S1 evidence: "Other sources of time are not monitored or certified by NIST."
• id: C3 claim: UTC(NIST) is the U.S. national standard for time-of-day, time interval, and frequency. source: S2 evidence: "UTC(NIST) is the U. S. national standard for time-of-day, time interval, and frequency."

Sources

• [S1] NIST Internet Time Service (ITS). NIST. https://www.nist.gov/pml/time-and-frequency-division/time-distribution/internet-time-service-its . License: US government work, public domain. Accessed 2026-11-04.
• [S2] Time Realization and Distribution Group. NIST. https://www.nist.gov/pml/time-and-frequency-division/time-services . License: US government work, public domain. Accessed 2026-11-04.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted).

@misc{iil_note_cgjbk23e,
  author = {{iinventedlight}},
  title = {{NIST time services: how official U.S. time is distributed}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-04},
  url = {https://iinventedlight.com/library/notes/nist-time-services/},
  note = {Permanent ID iil:note:cgjbk23e. License: CC-BY-4.0.},
}

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