UTC Continuity Approved: Why Leap Seconds Are Being Phased Out
The General Conference on Weights and Measures has published Resolution 5, "On the technical actions needed to ensure the continuity of UTC" BIPM Resolution 5. It is the implementation step for a decision taken four years earlier to loosen the tight coupling between atomic time and Earth rotation, and to keep civil time continuous for at least a century.
The decision was finalized at the 28th meeting of the Conference, held from Tuesday 13 to Thursday 15 October 2026 at the Palais des Congrès de Versailles CGPM meeting page. The Conference is the plenary and policy-making body of the International Bureau of Weights and Measures and its supreme authority, made up of representatives of Member States. It meets every four to six years to decide scientific, institutional and financial matters for the bureau and its work.
The baseline was set at the 26th meeting in 2018. In Resolution 2 that year, the Conference confirmed that UTC is produced by the bureau at the same rate as TAI, International Atomic Time, the steady scale from atomic clocks. UTC differs from TAI only by a whole number of seconds. The same resolution stated that UTC is the only recommended time scale for international reference and the basis of civil time in most countries.
The pivot came at the 27th meeting in 2022. In Resolution 4 on the use and future development of UTC, the Conference decided that the maximum allowed value for the difference |UT1-UTC| will be increased in, or before, 2035, to ensure the continuity of UTC for at least a century. UT1 is the time scale that tracks Earth's angular rotation. The Conference also asked the International Committee for Weights and Measures to draft a resolution for the 28th meeting in 2026 to put that decision into effect.
That whole-second offset exists because UTC is kept in agreement with UT1. As long as |UT1-UTC| is held to a fraction of a second, the drift between steady atomic ticking and an irregular planet has to be corrected with leap seconds. A positive leap second inserts a second. A negative leap second would suppress one.
The risk driving the change is the second case. A negative leap second has not previously been applied. A March 2025 workshop organized by the Consultative Committee for Time and Frequency and the International Earth Rotation and Reference Systems Service estimated that the probability of a negative leap second will increase rapidly, reaching 30% probability by 2035.
Resolution 5 addresses the surrounding technical infrastructure rather than redefining time itself. It notes ITU Resolution 655 (REV.WRC-23), which set a transition period to allow the increased difference between UT1 and UTC to be sent over radiocommunication systems until 2035, but no later than 2040 where existing equipment cannot be replaced earlier. It notes work in the ITU Radiocommunication Sector, where Working Party 7A is studying possible updates to the code for transmission of the UT1-UTC difference by radio stations.
For systems that need Earth orientation, the resolution points to existing distribution channels. Users of UT1 can find UT1-UTC information in GNSS navigation messages and in internet services provided by the Earth rotation service, National Metrology Institutes and other services. Most networked infrastructure can stay on continuous UTC. The smaller set of applications that need UT1, from telescope pointing to certain space and geodetic operations, retrieves that correction, known as DUT1, separately.
Procedurally, the continuity proposal moved through 2026 as Draft Resolution C. The bureau published Version 5 of the Draft Resolutions for the 28th meeting, updated in July 2026, alongside a dedicated report on Draft Resolution C. The draft as published before the meeting envisaged entry into force from 20 May 2027 BIPM time and future page. The convocation also listed work on an international lunar reference time scale and its traceability to UTC alongside the UTC continuity item.
In my view, the practical interest for technology teams lies less in metrology and more in failure modes removed. Leap-second insertion has long been a source of edge cases in kernels, distributed databases, financial timestamping and synchronization protocols. A negative leap second would be worse because it is untested in production at scale. Increasing the allowed |UT1-UTC| does not remove the divergence between atomic time and rotation. It changes where that divergence is handled.
One point worth flagging is that the burden shifts explicitly toward UT1 consumers. If UTC is allowed to drift further from UT1, code that assumed |UT1-UTC| stayed under 0.9 seconds will need review. Broadcast formats, parsers for DUT1, and documentation that hard-codes a small tolerance are the obvious places to look. The ITU transition window to 2035, with limited extension to 2040 for equipment that cannot be replaced earlier, gives vendors and operators a defined period to update receivers and software, but embedded radio time-code handling tends to live long in the field.
The broader context here is continuity as a design choice. UTC remains the single international reference for civil time. TAI continues underneath it at the same rate. What changes is how often civil time is stepped to follow the planet. For a technology stack built on the assumption that time moves forward without repeated breaks, fewer steps and no backward step simplifies testing, reduces the need for special smearing or stepping logic, and lets GNSS and network time services distribute a stable reference while UT1 data travels by parallel channels for those that need it.
