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When is the next leap second, and what happens next

Key facts

  • No next leap second has been announced. The IERS declared none for the end of June 2026 and none for the end of December 2026. [1]Official
  • TAI minus UTC has stayed at 37 seconds since 1 January 2017, so UTC minus TAI is −37 seconds. [1]Official
  • Decided in 2022: Resolution 4 of the 27th CGPM states the maximum value for the difference UT1 minus UTC will be increased in, or before, 2035. [2]Official
  • Proposed in 2026: Decision CIPM/115-5 fixes 20 May 2027 as the implementation date in an updated Draft Resolution C, which goes to the 28th CGPM. [3]Official
  • The 28th CGPM meets on 13–15 October 2026. Until it votes, Draft Resolution C is a proposal. [4]Official

There is no next leap second. None has been announced for the end of June 2026, and none for the end of December 2026 [1]Official.

The difference TAI − UTC has stayed at 37 seconds since 1 January 2017, the date the most recent insertion took effect [1]Official. Written the other way round, UTC − TAI is −37 s [1]Official.

The short answer

Nobody can name a date, and nobody is meant to. A leap second follows a measurement of the Earth’s rotation rather than a rule, so the IERS declares one for a given June or December only when the measurement calls for it [1]Official.

The announcement arrives through the IERS Bulletin C. The data file behind it is updated through IERS Bulletin 72, issued in July 2026, and it expires on 28 June 2027 [1]Official.

So the honest answer is: no date exists today. What does exist is a decision about the future of the practice, and a separate proposal about when that decision takes effect.

Decided, proposed, still open

These three columns are the whole story, and mixing them is the usual mistake in press coverage.

Question Where it stands What the document says Source
Will the limit on UT1 − UTC grow Decided in 2022 Resolution 4 of the 27th CGPM decides that the maximum value for the difference UT1 − UTC will be increased in, or before, 2035 [2]Official
When the change takes effect Proposed, not voted Decision CIPM/115-5 of 2026 endorses an updated Draft Resolution C fixing 20 May 2027 as the implementation date [3]Official
How large the new limit is Open no figure appears in Resolution 4 or in the published CIPM decisions [2]Official
Why an earlier date is discussed Recorded in 2023 Decision CIPM/112-14 notes that the trend projected by the IERS would lead to a negative leap second for the first time before 2035 [3]Official
When the next leap second falls Open no announcement for June 2026 or December 2026 [1]Official

What the 2022 decision actually said

Resolution 4 of the 27th CGPM, in 2022, decides that the maximum value for the difference UT1 − UTC will be increased in, or before, 2035 [2]Official.

The same resolution gives the CIPM three jobs: propose a new maximum value that keeps UTC continuous for at least a century, prepare a plan to implement it by, or before, 2035, and draft a resolution for the 28th meeting of the CGPM in 2026 [2]Official.

That resolution is the part already agreed. Note what it does not contain: a number for the new limit, and a date earlier than 2035 [2]Official.

What is on the table in October

Decision CIPM/115-5 of 2026 endorses the proposal of the CCTF to update Draft Resolution C, on the technical actions needed to ensure the continuity of UTC, for presentation at the 28th meeting of the CGPM. The update fixes 20 May 2027 as the implementation date of the new maximum value for the difference UT1 − UTC [3]Official.

The 28th CGPM meets on 13–15 October 2026 [4]Official. Until the delegates vote, this is a draft: no date is fixed in a resolution of the CGPM, and leap seconds continue under the present rules [4]Official.

The committee that prepared the text is the CCTF, which handles time questions for the metre convention [5]Official. How a CGPM vote works, and what the different outcomes would mean, is covered in how CGPM voting works.

The chain of documents from the 2022 decision to the proposed date of 20 May 2027, and what each one settled, is laid out in leap seconds and the October vote [3]Official.

Why an earlier date is being proposed

Decision CIPM/112-14 of 2023 notes that the trend projected by the IERS would lead to a negative leap second for the first time before 2035, and asks the CCTF to identify the best way forward together with the ITU [3]Official.

A negative leap second has never happened. Every entry in the IERS file since 1972 raises TAI − UTC by one second; none lowers it [1]Official. Removing a second would be new for every system that handles time.

Scientific American reports an estimate behind the earlier date: waiting until 2035 carries a 30 percent risk of a negative leap second [6]. That estimate is reported by the press, not printed in the CIPM decisions.

The number the press uses, and why we do not

Reports describe the proposal as replacing the leap second with a leap hour [6]. The size of the new limit is a reasonable guess, but it is not in the documents.

Neither Resolution 4 nor the published CIPM decisions give a figure for the new maximum [2]Official. We therefore print no figure either, and we will print one when a document carries it.

What to do with this today

If you timestamp data, nothing changes yet. UTC still carries whole leap seconds, TAI − UTC is still 37 s, and any offset you hard-code should be read from the IERS file rather than remembered [1]Official.

The complete series, with the date and the offset after each insertion, is in every leap second since 1972 [1]Official. What the mechanism is, and who announces it, is in what is a leap second.

For the lunar side of the same meeting, the separate lunar item on the October agenda is read in Draft Resolution D explained, and every lunar figure we publish is a model value derived on the methodology page [7]. The standards tracker carries the dated milestones, and the Learn hub lists the rest of our articles.

Sources

  1. Value of TAI-UTC (IERS leap second data file) — International Earth Rotation and Reference Systems Service — Earth Orientation Centre, Observatoire de Paris, . Official document. Verified .
  2. Resolution 4 of the 27th CGPM (2022): On the use and future development of UTC — Bureau International des Poids et Mesures — 27th General Conference on Weights and Measures, . Official document. Verified .
  3. Meetings of the CIPM: Outcomes — Bureau International des Poids et Mesures, . Official document. Verified .
  4. Convocation of the 28th meeting of the CGPM (O/Ref AK-CFA-2026-1001) — Bureau International des Poids et Mesures / CIPM, . Official document. Verified .
  5. Latest Updates from 2025 CCTF Meeting — Bureau International des Poids et Mesures, . Official document. Verified .
  6. Exclusive: International timekeepers to vote on changing the leap second to a leap hour — Scientific American (Emma Gometz, edited by Claire Cameron), . Press. Verified .
  7. Our calculation: the lunartime.org clock model — Lunartime Editorial, . Our calculation. Verified .

Last verified