International Timekeepers Vote to End Leap Second; Continuous UTC to Start May 20, 2027

International timekeepers have formally voted to retire the leap second, ending the longstanding practice of periodically adjusting Coordinated Universal Time to stay closely aligned with Earth’s rotation. The 28th General Conference on Weights and Measures, the member-state body that adopts global metrology resolutions through the International Bureau of Weights and Measures, adopted Resolution 5 on Thursday. The resolution says “continuous UTC will become effective on 20 May 2027.”

What changes is the rule governing how far civil time can drift from Earth-rotation time. UTC, the world’s civil time standard, is based on atomic clocks. UT1 tracks Earth’s actual rotation. Under the old system, leap seconds were inserted to keep UTC within about plus or minus 0.9 second of UT1. Under the new rule, that tight link is gone: Resolution 5 says “the maximum value for the difference |UT1-UTC| will be 3 600 seconds (1 hour), ensuring the long term continuity for UTC for several centuries.” That does not mean clocks will jump by an hour next year. The one-hour figure is a new upper limit, not a scheduled adjustment on May 20, 2027.

The practical reason for the change is that modern digital systems have found leap seconds increasingly difficult to handle. Telecommunications networks, satellite navigation, software systems, financial infrastructure and other timing-dependent technologies can be disrupted by one-second discontinuities. Timekeeping experts have argued that the risks are even greater if the world were to face its first negative leap second — a change that would require skipping a second rather than repeating one.

That risk was no longer hypothetical in the technical case presented to governments. A July 2026 report from the Consultative Committee for Time and Frequency, a technical body at the BIPM, said: “A ... common model ... led to the estimation of a ∼ 30 % probability of a negative leap second in the next 10 years, together with a ∼ 50 % probability of a positive leap second.” A negative leap second has never been implemented.

The leap-second system dates to 1972. Since then, experts have inserted 27 positive leap seconds into UTC, with the last added on Dec. 31, 2016. The International Earth Rotation and Reference Systems Service, the organization that monitors Earth rotation and announces leap seconds, said in Bulletin C 72 on July 6 that “NO leap second will be introduced at the end of December 2026.” It also said UTC-TAI = -37 s has been in effect since Jan. 1, 2017.

In effect, the decision means there will no longer be routine leap-second insertions under the old rule going forward. Instead, UTC will continue as a continuous atomic-time scale for civil use, while UT1 will remain the measure of Earth’s actual rotation. For most people, nothing visible will happen when the new regime takes effect in 2027: phones, computers and wall clocks will not suddenly shift by an hour.

The significance is longer term. The world’s official civil time standard is being moved onto a continuity-first model, one designed to avoid abrupt one-second steps that technical experts now see as more dangerous than a very gradual divergence from solar time. Even with the new one-hour ceiling, any drift on that scale would unfold over centuries under typical projections, not on a human-news timetable next spring.