Scientists have voted to abolish the leap second, ending more than half a century of second-by-second corrections to the world's clocks. Representatives from 69 countries voted on Thursday 15 October 2026 at a meeting of the International Bureau of Weights and Measures (BIPM) in Versailles, France, settling a debate that physicists have been having for the past 30 years, according to New Scientist.
The leap second will be replaced by a "leap hour". Instead of inserting occasional single-second corrections to keep atomic time aligned with the Earth's rotation, Coordinated Universal Time (UTC) will be allowed to drift away from astronomical time for up to an hour before being corrected. The change takes effect on 20 May 2027.
The vote reflects how deeply precise timekeeping is now embedded in modern life. What began as an astronomers' adjustment has become a potential hazard for the systems that run the planet.
What the Versailles vote decided
Leap seconds were introduced in 1972 to keep universal clock time in sync with the Earth's rotation. Twenty-seven have been added to UTC since then, each one a tiny, carefully coordinated event designed to stop atomic time and sky-based time drifting apart.
From 20 May 2027 that system ends. Under the new arrangement, UTC will be permitted to diverge from astronomical time for up to an hour before a "leap hour" correction is applied. Noël Dimarcq, head of the BIPM committee on time and frequency, said of the decision: "The changes will have a profound impact on future timekeeping both on the earth and other planets."
Why leap seconds became a liability
Technology now depends on timing with a precision that would have been unthinkable in 1972. Stock markets, telecommunications networks and GPS all rely on precisely synchronised clocks, and each leap-second insertion carries risk for mobile networks and electricity grids.
The danger is not theoretical. Previous leap seconds have crashed websites and disrupted airline booking systems. As Emily Akkermans, curator of time at the Royal Observatory Greenwich, put it: "Time is now so much more important and integrated into these systems. What once worked has become more problematic." Even the market data that underpins modern trading depends on timestamps accurate to fractions of a second.
The debate gained fresh urgency because Earth's rotation has recently begun to accelerate slightly, raising the prospect of a first-ever negative leap second, a backward jump in time whose practical consequences are hard to predict.
What a leap hour actually means
In practice, the new system means civil time will slowly slide away from the Sun's position in the sky. Emily Akkermans described the slow effect of the drift as "like living on daylight savings time, but permanently", with noon gradually moving away from the moment the Sun is highest.
Nobody alive today is likely to notice. Noël Dimarcq said it will take several centuries before a leap-hour correction is actually needed, while Setnam Shemar of the UK's National Physical Laboratory said: "It'll be many hundreds or even thousands of years before a leap hour is needed."
Astronomers, who need to know exactly where the Earth is pointing, will continue to get what they need. Shemar noted that physicists will still accurately track Earth's rotation for astronomy and celestial navigation, even as civil time stops being corrected second by second.
A debate 30 years in the making
For most of history, clocks were set by the apparent position of the Sun. That changed in 1967, when physicists defined Coordinated Universal Time from the vibration frequency of the caesium-133 atom, a definition far steadier than the turning Earth.
Because the planet's rotation is irregular and has generally been slowing, periodic corrections were needed to keep the atomic definition matched to astronomical time, and the leap second was born in 1972. Physicists have argued about a replacement for the past 30 years. The Versailles vote finally settled it with the leap-hour concept.
What else timekeepers agreed
The meeting went beyond the leap second. The BIPM also asked for proposals on a new definition of the second itself, because optical clocks now outperform the caesium clock on which the 1967 definition rests. A redefinition would be the first since atomic time began.
Looking further afield, the meeting called on international agencies to consider a universal time standard for the Moon, a practical necessity as lunar missions multiply and a permanent human presence moves closer to reality.
What happens between now and May 2027
With the decision made, the work shifts to implementation. Watch for guidance from the BIPM, national laboratories such as the UK's NPL, and tech standards bodies on how operating systems, telecommunications and financial infrastructure should handle the transition by 20 May 2027.
Also watch for reaction from cloud providers and GPS operators who must implement the change, and from the small but vocal community of astronomers who opposed it.
FAQs
What is a leap second?
A leap second is an extra second occasionally inserted into Coordinated Universal Time (UTC), the world's agreed clock time, to keep it aligned with the Earth's rotation. Because the planet's spin is irregular and has generally been slowing, atomic clocks and astronomical time drift apart. Since the system was introduced in 1972, 27 leap seconds have been added to UTC.
Why are scientists abolishing them now?
Precise timing is now woven into modern infrastructure, from stock markets and telecommunications to GPS, making each insertion a risk to mobile networks and electricity grids. Past leap seconds have crashed websites and disrupted airline booking systems. The debate has run for 30 years, but the prospect of a first-ever negative leap second, as Earth's rotation recently began accelerating slightly, added urgency.
What is a 'leap hour' and when will it be needed?
Under the new system, UTC will be allowed to drift away from astronomical time for up to an hour before a correction is applied. Because the drift is so gradual, Noël Dimarcq of the time and frequency committee said a correction will not be needed for several centuries, while Setnam Shemar of the UK's National Physical Laboratory said it could be many hundreds or even thousands of years away.
Will anything change for my clocks in May 2027?
No. The change takes effect on 20 May 2027 but will have no visible effect on your watch, phone or computer. The slow divergence between atomic time and solar time is imperceptible in daily life. Physicists will still track Earth's rotation precisely for astronomy and celestial navigation; only civil time stops being corrected second by second.
What is happening with the definition of the second?
The second has been defined since 1967 by the vibration frequency of the caesium-133 atom, but newer optical clocks are now far more accurate. At the same Versailles meeting, the BIPM asked for proposals on a new definition of the second. It also called on international agencies to consider a universal time standard for the Moon, as lunar missions and a permanent presence grow.
🤖 This article was rewritten by Feed and Figures' editorial AI from a report originally published by New Scientist. Facts and quotes are preserved from the original; the rewrite focuses on clarity and structure. For the unedited original, see the source link below.
