Representatives from 69 countries have voted to abolish leap seconds, the periodic adjustments used since 1972 to keep atomic clocks synchronized with Earth's rotation. The decision, made on 15 October at a meeting of the International Bureau of Weights and Measures (BIPM) in Versailles, France, will replace leap seconds with a «leap hour» system starting 20 May 2027.
Under the new approach, Coordinated Universal Time (UTC) will be allowed to drift up to an hour from astronomical time before a correction is applied. This marks a fundamental shift in how the world defines time, moving away from a system that has required 27 leap seconds over the past five decades.
For most of history, clocks were set by the apparent position of the sun. But in 1967, after the development of super-accurate atomic clocks, physicists defined UTC based on the vibration of caesium-133 atoms. Natural fluctuations in Earth's rotation meant periodic corrections were needed. As the planet's rotation slowed, extra seconds were added to UTC to keep it in sync with the sky.
That strategy is now being abandoned, partly because modern technological systems — from stock markets to telecommunications to GPS — have become far more reliant on precise, uninterrupted timekeeping. Introducing a leap second is a complex operation that can disrupt these systems. Previous leap seconds have caused websites to crash and disrupted airline booking systems, according to Setnam Shemar of the National Physical Laboratory in Teddington, UK.
«Time is now so much more important and integrated into these systems,» said Emily Akkermans, curator of time at the Royal Observatory in Greenwich, London. «What once worked has become more problematic.»
Adding to the urgency, Earth's rotation has recently begun to accelerate slightly, raising the daunting possibility of a first-ever negative leap second — a jump backwards in time with unpredictable consequences. Noël Dimarcq, who heads the BIPM's committee on time and frequency, noted that physicists have debated a replacement for the leap second for 30 years.
The leap hour should ensure an uninterrupted flow of time for «several centuries,» Dimarcq said. Shemar emphasized that the effects involved are very small: «It'll be many hundreds or even thousands of years before a leap hour is needed.» Physicists will continue tracking Earth's rotation for astronomy and celestial navigation.
The BIPM meeting also requested proposals for a new definition of the second, needed because optical clocks are now far more accurate than the caesium clock currently underpinning time. It further called on international agencies to begin considering how to define a universal time standard for the moon.
Overall, the changes «will have a profound impact on future timekeeping both on Earth and other planets,» Dimarcq said. The shift to a leap hour represents a pragmatic compromise: rather than constantly adjusting atomic time to match a slightly irregular planet, the world will let the two diverge within a controlled limit. For most people, the change will be imperceptible, but for the infrastructure that depends on split-second precision, it removes a recurring source of risk.
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