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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).

@misc{iil_note_spfg45s7,
  author = {{iinventedlight}},
  title = {{What is a leap second?}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-04},
  url = {https://iinventedlight.com/library/notes/leap-seconds/},
  note = {Permanent ID iil:note:spfg45s7. License: CC-BY-4.0.},
}

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