28th General Conference on Weights and Measures in Versailles: from 20 May 2027 UTC runs without leap seconds, and a proposal for a new definition of the second is due at the 2030 conference
The General Conference on Weights and Measures (CGPM) met in Versailles on 13–15 October 2026 and adopted seven resolutions. The most important for time measurement, Resolution 5, makes continuous UTC effective from 20 May 2027 — without inserting leap seconds — and sets the maximum difference between astronomical time UT1 and UTC at 3,600 seconds, i.e. one hour.
- Resolution 5 (“On the technical actions needed to ensure the continuity of UTC”) recognises that industrial users and the synchronisation needs of national critical infrastructures, for example transportation, require continuous UTC without any future adjustment; the 3,600 s limit is to ensure the continuity of UTC for several centuries. The conference cites a March 2025 workshop of the Consultative Committee for Time and Frequency (CCTF) and the IERS: the probability of the first-ever negative leap second would increase rapidly, reaching 30% by 2035 — and such an adjustment has never been applied. Since 1972, 27 leap seconds have been inserted, the last at the end of 2016.
- The International Bureau of Weights and Measures (BIPM) is to collaborate with the ITU, IAU, IUGG, IERS and other organisations to inform users and support implementation, to monitor and model the UT1−UTC difference and regularly report its trend to the International Committee for Weights and Measures (CIPM), and, together with Regional Metrology Organizations, national metrology institutes and users, to promote continuous UTC and its timely provision.
- Resolution 4 (“On the future definition of the second”) requests the CIPM to work towards a proposal for the new definition of the second to be presented at the 29th meeting of the CGPM in 2030, together with a proposal for the date of its implementation, and encourages Member States to provide the human and financial resources needed to fulfil the mandatory criteria. According to the preamble, some criteria — such as the contribution of optical frequency standards to the calibration of TAI — are not yet met but could realistically be met by 2030.
- Resolution 6 concerns an international lunar reference time scale and its traceability to UTC: the starting point is Lunar Coordinate Time (TCL), defined by the International Astronomical Union in 2024, and space agencies are encouraged to fully describe the relativistic model, ephemerides and uncertainties when comparing a realisation of lunar time with UTC. Resolution 3 encourages the CIPM to extend the SI Reference Point into a comprehensive and authoritative digital framework for SI units, and invites national metrology institutes and other stakeholders to advance machine-actionable digital information and advanced digital certificates. The remaining resolutions concern the BIPM dotation for 2028–2031, universal adherence to the Metre Convention and the Rules of Procedure of the CGPM.
Source: BIPM — 28th meeting of the CGPM (13–15 October 2026), Resolutions 1–7