---
source: https://iinventedlight.com/library/notes/leap-seconds.md
title: "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, Leap Seconds FAQs, 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: "facts-general: timekeeping"
tags: time, utc, leap-second, standards
sources:
  - id: S1
    title: "Leap Seconds FAQs"
    publisher: NIST
    url: https://www.nist.gov/pml/time-and-frequency-division/leap-seconds-faqs
    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 a leap second?

## Short answer
A leap second is a second added to Coordinated Universal Time (UTC) so that atomic clock time stays in step with Earth's slightly irregular rotation. [S1]

## Key points
- UTC is based on atomic clocks, which are steadier than Earth's spin. [S1]
- Leap seconds have always come at the end of June or December, on the last second of the UTC day. [S1]
- All so far have been positive (a second added); a negative one is allowed but has never been used. [S1][S2]
- The first was inserted on June 30, 1972. They keep UT1 minus UTC within ±0.9 s. [S2]
- TAI (International Atomic Time) is currently 37 seconds ahead of UTC. [S2]

## Claims
- id: C1
  claim: A leap second is added to UTC to keep it synchronized with astronomical time.
  source: S1
  evidence: "A leap second is a second added to Coordinated Universal Time (UTC)"
- id: C2
  claim: Leap seconds have occurred at the end of December or June.
  source: S1
  evidence: "Leap seconds have always occurred at the end of December or the end of June"
- id: C3
  claim: The first leap second was inserted on June 30, 1972.
  source: S2
  evidence: "The first leap second was inserted into the UTC time scale on June 30, 1972."
- id: C4
  claim: The current difference between UTC and TAI is 37 seconds.
  source: S2
  evidence: "The current difference between UTC and TAI is 37 seconds."

## Sources
- [S1] Leap Seconds FAQs. NIST. https://www.nist.gov/pml/time-and-frequency-division/leap-seconds-faqs . 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).