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CGPM sets 20 May 2027 for continuous UTC, ending leap seconds

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The General Conference on Weights and Measures (CGPM), meeting for the 28th time in 2026, adopted Resolution 5, titled "On the technical actions needed to ensure the continuity of UTC". Its two decisions are short. Continuous UTC will become effective on 20 May 2027. And the maximum value of the difference |UT1-UTC| will be 3,600 seconds (1 hour), which the text says ensures the long term continuity of UTC for several centuries. The decision clause does not use the words "leap second"; the recitals make the meaning clear, since industrial users and critical infrastructure "require UTC to be continuous without any future adjustment".

The resolution builds on earlier CGPM steps. In 2018 (26th meeting, Resolution 2) the CGPM confirmed that UTC is produced by the International Bureau of Weights and Measures (BIPM), runs at the same rate as International Atomic Time (TAI) and differs from it only by a whole number of seconds, and called UTC the only recommended time scale for international reference and the basis of civil time in most countries. In 2022 (27th meeting, Resolution 4) it decided that the maximum |UT1-UTC| difference would be increased in, or before, 2035, to keep UTC continuous for at least a century, and asked the CIPM to draft a resolution for the 2026 meeting. Resolution 5 is that resolution.

The text gives several reasons for acting now. A workshop run by the Consultative Committee for Time and Frequency (CCTF) and the IERS in March 2025, with Earth-rotation experts, estimated that the probability of a negative leap second will rise rapidly, reaching 30 % by 2035. A negative leap second has never been applied, and it is considered to pose a high risk of anomalies and disruption to critical infrastructure that is largely unprepared. Preparing for one would require financial investment from industries that rely on time synchronization, estimated to be similar to their preparations for the millennium bug. The text also says the growing influence of IoT, dispersed multi-sensor systems, smart grids and virtual services raises the risk of malfunction with any inserted leap second, especially a negative one. And global service providers use different mitigation methods for leap seconds in their server systems to avoid time steps, which leads to inconsistent timing between systems and in disseminated time and frequency, which do not conform to the SI unit.

Support came from several bodies. ITU-T, URSI, working groups such as IEEE 1588 and individual industry representatives gave the BIPM formal statements on the need to bring forward continuous UTC to avoid a negative leap second. The resolution also warns that a delay could cause some user organizations to switch to a reference such as GNSS time instead of UTC, seriously compromising international standardization in metrology. It adds that some applications need a fixed maximum |UT1-UTC| to design operational software, and notes that in many countries civil time already differs from local solar time by more than one hour because of time zones, with daylight-saving changes of an hour introduced regularly.

On the radio side, the ITU Radiocommunication Sector, through Working Party 7A, is studying updates to the code that radio stations use to transmit UT1-UTC. ITU Resolution 655 (REV.WRC-23) allows a transition period for disseminating the increased difference until 2035, but no later than 2040, where existing equipment cannot be replaced earlier. The CGPM welcomes the BIPM-ITU-R collaboration and the IERS commitment to take all necessary measures to change its operational provision for, and announcement of, leap seconds. Users of UT1 can still find the UT1-UTC value in GNSS navigation messages and in internet services from the IERS, national metrology institutes and others.

The resolution asks the BIPM to keep working with the ITU, IAU, IUGG, IERS and other affected organizations to inform users and support the application of the decision; to keep monitoring UT1-UTC and its estimated evolution as determined by the IERS; to report regularly to the CIPM; and to work with Regional Metrology Organizations, national metrology institutes and users to promote understanding of why a continuous UTC matters. The page notes that the official version of the resolution is the French text.

Key facts

  • CGPM Resolution 5 (28th meeting, 2026) decides that continuous UTC becomes effective on 20 May 2027.
  • The maximum |UT1-UTC| difference will be 3,600 seconds (1 hour), which the text says ensures UTC continuity for several centuries.
  • A March 2025 CCTF/IERS workshop estimated a 30 % probability of a negative leap second by 2035; none has ever been applied.
  • Preparing for a negative leap second would cost industries an estimated amount similar to their millennium bug preparations.
  • ITU Resolution 655 (REV.WRC-23) allows radio dissemination of the increased UT1-UTC difference until 2035, but no later than 2040, where equipment cannot be replaced earlier.

Why it matters

UTC is the basis of civil time in most countries, and the resolution changes how it is kept. The CGPM's 2022 decision to enlarge the allowed UT1-UTC gap by 2035 is now made concrete: continuous UTC from 20 May 2027 and a 3,600-second limit. The stated driver is risk. Earth's rotation could force a negative leap second, which has never been applied and is considered likely to disrupt critical infrastructure that is largely unprepared. The text sets that against the cost of preparing for one, which it compares to the millennium bug.

Who it affects

The resolution names transportation, energy distribution, telecommunication and space projects as users that need UTC to be continuous without any future adjustment. It also points to IoT, smart grids, multi-sensor systems and virtual services, and to global service providers whose servers handle leap seconds in different ways. Radio operators are affected too: the code used by radio stations to transmit UT1-UTC is under study at ITU-R Working Party 7A. People who use UT1 itself will see a larger gap between UT1 and UTC over time.

How to use it

This is a standards decision, not a product. The resolution says users of UT1 can find the UT1-UTC value in GNSS navigation messages and in internet services from the IERS, national metrology institutes and others. It sets 20 May 2027 as the date continuous UTC takes effect and fixes 3,600 seconds as the design limit for |UT1-UTC|, which the text says some applications need in order to design operational software. It asks the BIPM, with the ITU, IAU, IUGG, IERS and others, to inform users and support application of the decision. Where radio equipment cannot be replaced earlier, ITU Resolution 655 allows the increased difference to be disseminated until 2035, but no later than 2040.

How solid is it

This is the primary text: the CGPM's own Resolution 5, with a DOI. It is an English translation, and the page says the official version is the French text. The decision clause says "continuous UTC will become effective on 20 May 2027"; the elimination of leap seconds follows from the recitals rather than from that clause. The 30 % figure comes from a March 2025 CCTF/IERS workshop estimate, not a measurement. The source gives no vote count, dissenting states or reactions of individual countries.

Risks and caveats

The source gives no mechanism for how UT1 and UTC will be kept within 3,600 seconds, or what happens once the limit is eventually reached beyond saying it ensures continuity for several centuries. It gives no exact hour or time of day for 20 May 2027, and no dollar figure for infrastructure preparation, only the millennium bug comparison. The resolution itself warns of a risk if the change is delayed: some user organizations could move to a reference such as GNSS time instead of UTC, seriously compromising international standardization in metrology. Radio dissemination of the larger difference also depends on a transition period that can run to 2035 or, at the latest, 2040.

“require UTC to be continuous without any future adjustment”

— CGPM Resolution 5 (2026), on the needs of industrial users and national critical infrastructures