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Short answer

Coordinated Universal Time (UTC) is the official, internationally agreed standard for world time. In the U.S., NIST keeps its own version, UTC(NIST), which is official U.S. time. Now and then one second is added to UTC, a leap second, to keep it within 0.9 seconds of UT1, a time scale based on Earth's rotation. [S1][S2]

Key points

• NIST time signals (WWV, WWVH, WWVB and the Internet Time Service) all follow UTC(NIST). [S2]
• NIST's predecessor first built the UTC(NIST) time scale in the 1960s. [S1]

Claims

• id: C1 claim: UTC is the official, internationally agreed standard for world time. source: S1 evidence: "is the official, internationally agreed upon standard for world time"
• id: C2 claim: NIST's UTC(NIST) provides official time for the United States. source: S1 evidence: "which provides official time for the United States"
• id: C3 claim: The added second is called a leap second. source: S2 evidence: "This second is called a leap second."
• id: C4 claim: Leap seconds keep UTC within 0.9 s of UT1. source: S2 evidence: "keep the UTC time scale within ±0.9 s of the UT1 astronomical time"

Sources

• [S1] UTC(NIST) Time Scale. NIST. https://www.nist.gov/pml/time-and-frequency-division/time-realization/utcnist-time-scale . License: US government work, public domain. Accessed 2026-11-04.
• [S2] Leap second and UT1-UTC information. NIST. https://www.nist.gov/pml/time-and-frequency-division/time-realization/leap-seconds . 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_8kdkbaht,
  author = {{iinventedlight}},
  title = {{What is UTC, and what is a leap second?}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-04},
  url = {https://iinventedlight.com/library/notes/utc-explained/},
  note = {Permanent ID iil:note:8kdkbaht. License: CC-BY-4.0.},
}

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