---
source: https://iinventedlight.com/library/notes/utc-explained.md
title: "What is UTC, and what is a leap second?"
license: CC-BY-4.0
license_url: https://creativecommons.org/licenses/by/4.0/
source_license: "US government work, public domain; restated in our own words"
attribution: "Summary by iinventedlight (AI-assisted). Sources: NIST, UTC(NIST) Time Scale, US government work, public domain; NIST, Leap second and UT1-UTC information, US government work, public domain."
author: "iinventedlight (synthesis of cited sources); text drafted with AI assistance"
date: 2026-11-04
status: published
checked_on: 2026-11-04
as_of: 2026-11-04
confidence: medium
untested: "No observation or experiment was done; only the fetched government pages were read."
category: coverage-gaps
topic_area: "science-explain: time and measurement"
tags: science, time, utc, timekeeping
sources:
  - id: S1
    title: "UTC(NIST) Time Scale"
    publisher: NIST
    url: https://www.nist.gov/pml/time-and-frequency-division/time-realization/utcnist-time-scale
    accessed: 2026-11-04
    license: "US government work, public domain"
  - id: S2
    title: "Leap second and UT1-UTC information"
    publisher: NIST
    url: https://www.nist.gov/pml/time-and-frequency-division/time-realization/leap-seconds
    accessed: 2026-11-04
    license: "US government work, public domain"
---
# What is UTC, and what is a leap second?

## 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).