Summary

  • The 28th CGPM makes continuous UTC effective on 20 May 2027 and fixes the long-term ceiling for |UT1−UTC| at 3,600 seconds.
  • WRC-23 allows radio systems to carry the increasing UT1−UTC difference through a transition ending in 2035, or no later than 2040 when existing equipment cannot be replaced earlier.

One decision, two clocks

The General Conference on Weights and Measures has put a date on a change that time engineers have debated for years: continuous Coordinated Universal Time takes effect on 20 May 2027. There will be no future leap-second insertion into UTC. The same resolution sets a maximum separation of 3,600 seconds between UTC and UT1, the time scale tied to Earth’s rotation, saying that limit supports UTC’s continuity for several centuries.

That is the governing decision. It is not the whole transition. UTC is the international reference; UT1 remains useful where Earth rotation matters. Users can obtain the UT1−UTC difference through GNSS navigation messages and services provided by the International Earth Rotation and Reference Systems Service, national metrology institutes and others. Radio has its own implementation problem: how to encode and transmit that growing difference to equipment already in service.

The radio calendar runs longer

WRC-23 Resolution 655 assigns ITU-R responsibility for the content and structure of time signals sent over radio, while BIPM maintains UTC. It allows a transition for disseminating the increased UT1−UTC difference until 2035 and, in cases where existing equipment cannot be replaced earlier, no later than 2040. The resolution also asks the Radiocommunication Bureau to report progress to WRC-27.

That is a conditional accommodation for equipment, not a second UTC start date and not a blanket permission for every service to postpone upgrades. The distinction matters. A receiver can keep working while its interpretation rules become wrong; a transmitter can remain on air while operators, users and downstream software disagree about what the signal means. A standards date does not update firmware by decree.

The split assigns work to different hands. CGPM sets the metrological decision. ITU-R develops radio-signal rules. Administrations and signal operators implement them. Manufacturers change receivers and decoders. Network, navigation, energy and other infrastructure operators decide which time reference each function actually requires. Users who need UT1 must preserve a traceable path to that difference even as UTC no longer receives leap-second steps.

A handbook, not a migration receipt

The timing is striking: on 30 October, ITU announced the 2026 edition of its Handbook on Selection and Use of Precise Frequency and Time Systems, its first update since 1997. The guide covers terrestrial radio and satellite dissemination, optical-fibre time transfer, optical atomic clocks, and “time as a service” with performance monitoring, resilience and traceability to UTC. ITU presents it as a technical reference for a system entering a new phase.

The breadth is useful, but a handbook is not evidence that an individual radio service has passed a compatibility test. The public resolutions reviewed here do not name a service-by-service migration roster, receiver population or national cutover schedule. A published inventory and compatibility test would change the readiness assessment. Until then, “continuous UTC starts in 2027” is precise; “the installed base will be ready” is not.

This is why the 2040 clause deserves a hard reading. It says legacy equipment may need time. It does not identify who owns each unit, who pays for replacement, who certifies the new decoding path or who bears the outage if a receiver mishandles the growing offset. The conference has settled the destination and left the field handoff to a chain of institutions and operators.