Leap seconds and the October 2026 vote
Key facts
- Resolution 4 of the 27th CGPM, in 2022, decides that the maximum value for the difference UT1 minus UTC will be increased in, or before, 2035. [1]Official
- Decision CIPM/115-5 of 2026 endorses an updated Draft Resolution C that fixes 20 May 2027 as the implementation date. [2]Official
- Decision CIPM/112-14 of 2023 records why an earlier date is discussed: the IERS trend would lead to a negative leap second for the first time before 2035. [2]Official
- The 28th CGPM meets on 13–15 October 2026, and Draft Resolution C is item 16 of the provisional agenda. [3]Official
- No leap second has been announced for June 2026 or December 2026, and TAI minus UTC has stayed at 37 seconds since 1 January 2017. [4]Official
Draft Resolution C is the leap second item at the 28th CGPM, scheduled for 13–15 October 2026 [5]Official. Its subject, as the convocation of 13 January 2026 puts it, is the continuity of UTC [3]Official.
On the technical actions needed to ensure the continuity of UTC
Press coverage tends to compress this into one sentence about abolishing the leap second. Two different things are being compressed, and they sit four years apart. One is decided. The other is proposed, and it is the part that goes to a vote in October.
What the 2022 resolution already decided
The direction was settled at the previous conference. Resolution 4 of the 27th CGPM, in 2022, states the decision in one clause [1]Official.
decides that the maximum value for the difference (UT1-UTC) will be increased in, or before, 2035
That clause fixes an outer limit, 2035, and nothing else. The same resolution then gives the CIPM three jobs, and the wording of each one matters for reading what came later [1]Official.
propose a new maximum value for the difference (UT1-UTC) that will ensure the continuity of UTC for at least a century
prepare a plan to implement by, or before, 2035 the proposed new maximum value
draft a resolution including these proposals for agreement at the 28th meeting of the CGPM (2026)
The third of those jobs is the reason a leap second draft reaches the 2026 meeting at all. The conference asked for a text in 2022, and the text arrives in October 2026 [1]Official.
What Draft Resolution C proposes
The update that goes to the conference is recorded as Decision CIPM/115-5 of 2026. It carries the only date in this story that is not 2035 [2]Official.
The CIPM endorses the proposal by the Consultative Committee for Time and Frequency (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|.
Read that decision closely. The CIPM endorses a proposal by the CCTF, and the proposal updates a draft for presentation to the conference [2]Official. A decision of the CIPM is not a resolution of the CGPM. Until the delegates vote, 20 May 2027 is a proposed implementation date and nothing more [3]Official.
The committee that made the proposal is the CCTF, which prepares time questions for the Metre Convention [6]Official. The procedure that would turn the draft into a resolution is set out in how the CGPM votes on a draft resolution.
Why the date moved earlier than 2035
The argument for acting before 2035 is on the record in an earlier decision. Decision CIPM/112-14 of 2023 gives the reason in a single sentence [2]Official.
The CIPM noted that the trend projected by the International Earth Rotation and Reference Systems Service (IERS) would lead to a negative leap second for the first time before 2035 and asked the CCTF to identify the best way forward and the necessary steps to allow a timely application of Resolution 4 adopted by the CGPM at its 27th meeting (2022) working together with the International Telecommunication Union (ITU).
A negative leap second has never happened. Every entry in the IERS data file since 1972 raises TAI − UTC by one second, and none lowers it [4]Official. Removing a second would be new code paths for every system that stores time, which is why the committees treat the possibility as a deadline rather than a curiosity.
Scientific American reports the estimate used to argue for a shorter timetable: waiting until 2035 carries a 30 percent risk of a negative leap second [7]. That figure comes from the press, not from the published decisions of the CIPM [2]Official.
The same reporting describes the change as a move from a leap second to a leap hour [7]. We do not print a size for the new limit. Neither Resolution 4 nor the published CIPM decisions state one, so there is no figure to cite [1]Official.
The chain of documents that leads to one date
Four documents stand between the 2022 decision and the October 2026 agenda [3]Official. The table takes them in order; the last column is the document each row rests on, and the third column says what the document settled and what it left to the next one.
| Step | Document, and its date | What it settled | What it left open | Source |
|---|---|---|---|---|
| 1 | Resolution 4 of the 27th CGPM, 2022 | The maximum value for UT1 − UTC is to be increased in, or before, 2035 | The size of the new maximum, and any date before 2035 | [1]Official |
| 2 | Decision CIPM/112-14, 2023 | The IERS trend would bring a negative leap second for the first time before 2035, so the CCTF works with the ITU on the way forward | What date would replace 2035 | [2]Official |
| 3 | Decision CIPM/115-5, 2026 | The CCTF proposal to update Draft Resolution C is endorsed, fixing 20 May 2027 as the implementation date in the draft | Whether the conference agrees | [2]Official |
| 4 | Convocation of 13 January 2026 | Draft Resolution C goes to the 28th CGPM as item 16, and the votes are item 25 | The outcome; the conference has not met | [3]Official |
Nothing in that chain is a leap second announcement. The practice today is unchanged: no leap second for the end of June 2026 and none for the end of December 2026, with TAI − UTC still 37 seconds [4]Official.
What stays open after the vote
Even a carried draft leaves three questions unanswered, and each one has a document that would have to answer it.
The size of the new maximum is the first. It appears in neither the resolution of 2022 nor the CIPM decisions of 2023 and 2026 [1]Official. A date of implementation without a value is a schedule, not a specification.
The second is the ITU. Decision CIPM/112-14 names the International Telecommunication Union as a partner in the work, because the dissemination of UTC in radiocommunication is its responsibility rather than the CGPM’s [2]Official.
The third is the behaviour of existing systems. No published decision says what operators of time distribution services are expected to do, or when they are expected to do it [2]Official. Until a value and a procedure are published, the practical advice in when is the next leap second still holds: read the offset from the IERS file rather than hard-coding it [4]Official.
Why this sits next to the lunar draft
Draft Resolutions C and D reach the same conference in the same week, C as item 16 and D as item 17 [3]Official. That is not a coincidence of scheduling. Both drafts are about what UTC is tied to.
Draft Resolution C concerns the tie between UTC and the rotation of the Earth, which is what the quantity UT1 − UTC measures [1]Official. Draft Resolution D concerns the tie between UTC and a proposed lunar reference time scale [3]Official. A conference that loosens one tie in the morning and defines another in the afternoon is answering one question twice.
The lunar half is read line by line in what Draft Resolution D proposes. Both items, and the third time draft on the definition of the second, are placed side by side in three time resolutions at the 2026 CGPM [3]Official.
Read next
The mechanism itself, and who announces an insertion, is in what is a leap second. The full series of 27 insertions, with the offset after each one, is in every leap second since 1972 [4]Official. The scale under discussion has its own glossary entry, UTC.
Both outcomes of the October votes, and the deadlines that hold either way, are set out in what happens after the vote. The dated milestones are on the standards tracker, the agenda item this draft sits on is tracked on our hub for the 28th CGPM, and the rest of our conference coverage is in the lunar time news hub.
Sources
- Resolution 4 of the 27th CGPM (2022): On the use and future development of UTC
- Meetings of the CIPM: Outcomes
- Convocation of the 28th meeting of the CGPM (O/Ref AK-CFA-2026-1001)
- Value of TAI-UTC (IERS leap second data file)
- 28th meeting of the CGPM (2026)
- Latest Updates from 2025 CCTF Meeting
- Exclusive: International timekeepers to vote on changing the leap second to a leap hour
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