17 OCT 2026 — Leap seconds will stop being added to Coordinated Universal Time (UTC) from 20 May 2027. Delegates at the General Conference on Weights and Measures (CGPM) in Versailles adopted the change on 15 October, ending a practice that has inserted 27 extra seconds since 1972. The same meeting declined, for now, to choose a new definition of the second itself, saying scientists have not reached a consensus.
RECATOOLS reported on the draft resolution in September, when the vote was still pending. The text passed as drafted.
What was decided
Resolution 5 of the conference states two decisions. Continuous UTC takes effect on 20 May 2027, and the permitted gap between clock time and the Earth's actual rotation, known as UT1 minus UTC, rises to 3,600 seconds, or one hour. The resolution says that ensures "the long term continuity for UTC for several centuries".
The resolution gives its reasoning: industrial users and critical infrastructure, including transport, energy, telecoms and space projects, need a time scale with no jumps. Recent changes in the Earth's rotation have also raised the prospect of a negative leap second, a minute of 59 seconds that has never been tried. A March 2025 workshop of Earth-rotation experts put the probability of one at 30 per cent by 2035, and the resolution warns that preparing for it would cost industry an amount "similar to their preparations for the millennium bug".
An hour of drift, eventually
An hour sounds large, but it will take a long time to build up. The resolution notes that people already live with bigger gaps: local clock time in many countries differs from solar time by more than an hour because of the choice of time zone, and daylight saving shifts clocks by an hour twice a year. How the gap will eventually be handled, if it ever approaches the limit, is left for later.
Fewer edge cases
The change also settles a practical mess. Global service providers have handled past leap seconds in different ways to avoid a sudden jump, which the resolution says has created inconsistencies between systems. For software that counts time in seconds since 1970, such as the Unix timestamps our timestamp converter reads, the end of leap seconds removes one source of edge-case bugs.
The second itself, still undecided
The second is currently defined by the vibration of caesium atoms. According to Resolution 4, optical clocks using other atoms now beat that standard's accuracy by a factor of up to 100. Scientists had hoped to move to an optical definition, but the conference recorded that "a consensus has not been reached" on whether to base it on one atomic species or an ensemble, or on which species to use. It also noted "noticeable inconsistencies" in some comparisons between optical clocks.
The conference asked the International Committee for Weights and Measures to bring a proposal to the next meeting, in 2030, along with a date for putting it into effect. The 2030 target remains alive, but some of the agreed conditions have not been met.
Time on the Moon
A third resolution looked further out. As space agencies plan navigation systems around the Moon, Resolution 6 warns that "a proliferation of reference time scales risks ambiguity" and must be avoided. Clocks on the Moon run at a slightly different rate because of relativity. The conference noted that studies show the International Astronomical Union's Lunar Coordinate Time can meet scientific needs without adjustment. It asked metrology institutes to work with space agencies so that any lunar time scale can be traced back to UTC.
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