BIPM Decides To Stop Adding Leap Seconds To UTC

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The General Conference on Weights and Measures has adopted a resolution making UTC continuous from 20 May 2027, ending future leap-second adjustments. The permitted difference between UTC and Earth-rotation time UT1 will rise to one hour; the BIPM says this is intended to reduce risks to systems that rely on synchronized time.

The General Conference on Weights and Measures (CGPM) has adopted a resolution to make Coordinated Universal Time (UTC) continuous from 20 May 2027, ending the insertion of leap seconds to keep civil time close to Earth’s rotation. The decision sets a new maximum difference of 3,600 seconds, or one hour, between UTC and UT1, the time scale tied to Earth’s rotation.

Resolution 5, adopted at the CGPM’s 28th meeting in 2026, directs the Bureau International des Poids et Mesures (BIPM) to implement continuous UTC on the specified date. UTC is produced by the BIPM at the same rate as International Atomic Time (TAI), but differs from it by an integral number of seconds. Leap seconds have been used to keep UTC aligned within set limits with UT1, which reflects the Earth’s changing rotation.

Under the new arrangement, the permitted absolute difference between UT1 and UTC will be capped at one hour. The resolution says this limit is intended to keep UTC continuous for several centuries. It does not mean that clocks will suddenly shift by an hour on 20 May 2027: the change removes the need for future leap-second adjustments, while the offset between UT1 and UTC is expected to evolve over time.

The resolution asks the BIPM to work with organizations including the International Telecommunication Union (ITU), the International Astronomical Union, the International Union of Geodesy and Geophysics, and the International Earth Rotation and Reference Systems Service (IERS). The BIPM is also asked to monitor the UT1–UTC difference and its projected evolution, and to report regularly to the International Committee for Weights and Measures.

Why Removing Leap Seconds Matters

The decision responds to the difficulty of applying leap seconds consistently across systems that depend on precise, shared timing. The resolution identifies telecommunications, energy distribution, transportation and space projects as examples of sectors whose infrastructure relies on synchronized time. It also points to the growth of connected devices, smart grids, distributed sensors and virtual services as factors increasing the risk of malfunctions when a leap second is introduced.

Leap seconds can be handled differently by service providers, according to the resolution. Some systems spread an adjustment over time rather than making a single step, a practice that can create timing inconsistencies between systems. A continuous UTC is intended to give operators and software designers a stable reference without future second-level insertions. That does not remove the need to track Earth’s rotation: users who need UT1 can obtain the difference from satellite navigation messages and services provided by the IERS, national metrology institutes and others.

The change matters beyond specialist timing networks because UTC underpins civil timekeeping and coordination across borders. The CGPM’s resolution frames continuity as a way to preserve UTC as the common international reference, rather than risk some organizations moving to other time scales if they consider UTC too difficult to use.

From the 2022 Decision to 2027

The CGPM had already set the direction for this change. In Resolution 4 of 2022, the conference decided that the maximum permitted difference between UT1 and UTC would be increased in or before 2035, with the aim of maintaining continuity for at least a century. It asked the International Committee for Weights and Measures to prepare a further resolution for the 2026 meeting.

Resolution 5 says a March 2025 workshop organized by the Consultative Committee for Time and Frequency and the IERS assessed the likelihood of a negative leap second rising rapidly, reaching a 30% probability by 2035. No negative leap second has previously been applied, and the resolution describes the possibility as a risk because critical infrastructure is largely unprepared for one. It also cites formal statements to the BIPM from international organizations and industry representatives supporting an earlier move to continuous UTC.

The policy change also needs coordination beyond metrology organizations. The ITU’s Radiocommunication Sector has been studying updates to how radio systems transmit the UT1–UTC difference. A World Radiocommunication Conference resolution allows for a transition period for systems that cannot be updated earlier, with dissemination of the increased difference possible until 2035, but no later than 2040, according to the CGPM text.

““continuous UTC will become effective on 20 May 2027””

— The 28th General Conference on Weights and Measures, Resolution 5

Remaining Work on Time Signals

The resolution establishes the policy and effective date, but implementation details remain subject to work by the organizations responsible for time dissemination and affected equipment. In particular, the ITU-R is studying possible updates to radio transmission codes for the UT1–UTC difference, while some legacy systems may need a longer period to accommodate the larger permitted offset.

The resolution does not specify how every national service, software provider or operator will update its systems, nor does it give a detailed schedule for those changes. It also does not predict the precise rate at which the UT1–UTC difference will grow. The BIPM is tasked with monitoring the value and its estimated evolution as modelled by the IERS. The cited 30% probability concerns the prospect of a negative leap second by 2035, as estimated at the 2025 workshop; it is not a prediction that such an adjustment will occur.

Preparing Systems Before May 2027

The stated next milestone is 20 May 2027, when continuous UTC is to become effective. Before then, the BIPM is to continue working with the ITU, IAU, IUGG, IERS and other affected organizations to inform users and support application of the decision. It will also track the changing UT1–UTC difference and report its development to the CIPM.

Radio-system updates and arrangements for equipment that cannot be replaced promptly remain part of the transition work. Users whose applications require UT1 will continue to need access to the difference between UT1 and UTC through satellite navigation messages and relevant time services. The resolution sets the international direction; the technical steps for individual systems will depend on the responsible operators and standards bodies.

Key Questions

What did the BIPM and CGPM decide?

The 28th CGPM adopted Resolution 5, setting 20 May 2027 as the date for continuous UTC. The resolution ends future leap-second adjustments and raises the permitted UT1–UTC difference to one hour.

Will clocks jump forward by an hour in 2027?

No. The one-hour limit is a maximum permitted difference between UTC and UT1, not a scheduled clock change. The resolution sets a future cap; it does not call for an hour-long adjustment on the effective date.

What is the difference between UTC and UT1?

UTC is the international time scale produced by the BIPM and used as a reference for civil time. UT1 is tied to Earth’s rotation. Leap seconds were used to keep the difference between them within limits.

Why end leap seconds?

The resolution says leap-second insertions can create operational risks and inconsistencies because systems handle them in different ways. It also cites concern that a negative leap second could be difficult for infrastructure that has not prepared for one.

How can users obtain UT1 information after the change?

The resolution says users can find the UT1–UTC difference in Global Navigation Satellite System navigation messages and through services provided by the IERS, national metrology institutes and other providers.

Source: hn