Time Metrology at the BIPM: Preparing for Future Decisions

Time Metrology at the BIPM: Preparing for Future Decisions

Context and information for the 28th CGPM on current developments in international timekeeping.

 

Time metrology at the BIPM

Global synchronization of time is essential for modern society. It enables everything from scientific research and satellite navigation to telecommunications, financial transactions and digital infrastructure to operate using a common, internationally agreed reference.

To support this, the BIPM Time Department works with National Metrology Institutes (NMIs) and international partners to realize, maintain and disseminate Coordinated Universal Time (UTC) and its rapid realization, UTCr.

Learn more about BIPM Time Metrology →

What’s next for international timekeeping?

The international metrology community is considering several developments that may influence the future of international timekeeping.

These topics are being addressed through work coordinated by the Consultative Committee for Time and Frequency (CCTF) and are reflected in draft resolutions prepared for consideration at the 28th meeting of the General Conference on Weights and Measures (CGPM) in October 2026.

Decisions under consideration at CGPM 2026

The summaries below provide background information on each topic under consideration. Draft resolutions should be consulted for the official wording.

Topic What is under consideration Further information
Future definition of the second

Context: The SI second is currently defined by the caesium-133 atom and realized by primary frequency standards with uncertainties approaching parts in 10−16.

However, advances in optical frequency standards are already producing devices that outperform the current primary realization of the SI second by almost two orders of magnitude.

As these technologies continue to mature, the international metrology community considering how the SI can continue to reflect advances in measurement science while maintaining continuity for users worldwide.

Proposal: Draft Resolution B requests that the CIPM continue the work needed to prepare a proposal for a new definition of the second based on optical frequency standards. The proposal would be presented to the 29th meeting of the CGPM (2030), together with a recommendation for when the new definition should be implemented.

The draft resolution also encourages National Metrology Institutes (NMIs) and national time laboratories to continue developing and comparing optical frequency standards, helping to ensure robust international measurement capabilities at uncertainties approaching 10−18.

 Draft Resolution B (2026) – On the future definition of the second (page 20)
Continuity of UTC

Context: UTC currently includes leap seconds to maintain its relationship with Earth's rotation.

As technology and user requirements evolve, the international time and frequency community is considering how UTC could evolve into a continuous time scale, in order to meet the needs of future users, while maintaining global coordination of timekeeping.

Proposal: Draft Resolution C proposes that the maximum value of |UT1−UTC| be increased to 3600 seconds (1 hour), therefore discontinuing the use of leap seconds. If agreed this will come into effect from 20 May 2027.

 Draft Resolution C (2026) – On the technical actions needed to ensure the continuity of UTC (page 22)
International lunar reference time scale

Context: Space agencies and international astronomical, geodesy and telecom organizations are studying the requirements for a common reference time scale to support future lunar positioning, navigation and timing services, including its relationship to UTC.

Proposal: Draft Resolution D recommends a way forward for the definition of an international lunar reference time scale and its traceability to UTC. This calls for continued collaboration between NMIs, the BIPM, space agencies and international organizations.

 Draft Resolution D (2026) – On the definition of an international lunar reference time scale and its traceability to UTC (page 25)

Hear more about these developments from experts

Explore expert presentations and explainers on current developments in time and frequency metrology.

Optical Clocks and the Next SI Second
On the Road to Redefining the Second
New Science and Defining the Second
Precision in Space: Why Metrology Matters for Satellites and Exploration
Why We Need to Redefine the Second and Time on the Moon
Optical Lattice Clocks - The Next Leap in Timekeeping

Supporting international coordination in timekeeping

The BIPM Time Department works with National Metrology Institutes and international partners worldwide to maintain international reference time scales and support globally coordinated timekeeping.

This work contributes to ensuring that time measurements remain comparable, traceable and internationally recognized, supporting applications in science, technology and infrastructure.

Explore the BIPM Time Department

Learn more about UTC, Rapid UTC, future developments in timekeeping and the work of the BIPM Time Department.

Return to the Time Department page →
This page was last updated in September 2026.