Scientists, diplomats and timekeepers voted in Versailles, France, to end the leap second and replace it with a leap hour in UTC, the world’s standard timescale. The change is set to take effect on May 20, 2027, and it would make the global clock with seconds run continuously instead of being nudged every few years.
The vote matters because UTC is the reference for computers, phones, boats, planes, satellites and GPS, and its reach is far beyond scientific circles. Noël Dimarcq, speaking moments before the decision, called the debate a 30-year effort and said, “After 30 years of debates, we are ready.”
UTC has been kept close to Earth’s rotation by adding leap seconds since 1972, when the first one was inserted. Since then, 27 leap seconds have been added, each one a small correction meant to keep UTC and UT1 within one second of each other. Under the new rule, that gap can widen to an hour, or 3,600 seconds, which is the sharp break with the system UTC was built to maintain.
That change was driven by a problem that has grown harder to manage. Scientists noticed soon after UTC was established in 1960 that Earth’s rotation was slowing slightly, but in the mid-2010s Earth’s spin began speeding up. By then, experts were already warning that a negative leap second could be needed as early as 2029, and engineers described that possibility as a mini Y2K event.
The leap-hour plan gives system makers more room, but it also leaves open the hard part: how to handle the transition across computers, telecommunications networks and navigation systems that have long relied on UTC being tied tightly to Earth-rotation time. Nick Stamatakos said avoiding negative leap seconds was a major reason to widen the window, and that is exactly what the vote does — it buys time by changing the rule, not by solving the underlying mismatch.
Since 2022, scientists, government officials, telecommunications professionals and others have been trying to settle the leap-second problem. The decision in Versailles closes one chapter of that effort and starts another, because UTC will no longer be held within one second of UT1. What remains is how the shift will be carried out once the continuous system begins on May 20, 2027.
