On Thursday, in a meeting room outside Paris, the world quietly changed the way it keeps time. Delegates at the 28th General Conference on Weights and Measures voted to abolish the leap second — the occasional extra second slipped into the world's clocks since 1972 — and replace it with something that won't be needed for a thousand years: the leap hour.
It is one of the most consequential decisions in the history of timekeeping, and almost nobody noticed. But the next time your phone, your bank, or your flight booking doesn't glitch for one mysterious second, this vote is why.
The vote that ended 54 years of extra seconds
The resolution was introduced by Noël Dimarcq, a metrologist at the Côte d'Azur Observatory in Nice and president of the Consultative Committee for Time and Frequency at the International Bureau of Weights and Measures (BIPM). Member states of the General Conference on Weights and Measures (CGPM) passed it by majority — not unanimously — at their meeting in Versailles on October 15, 2026.
The change takes effect on May 20, 2027 — World Metrology Day. From that date, Coordinated Universal Time (UTC) becomes continuous: no more inserted or deleted seconds, ever again.
The vote dramatically accelerates a plan the same body agreed in 2022, which had set 2035 as the target for widening the allowed gap between atomic time and Earth-rotation time. Thursday's resolution brings the change forward — and locks in the details.
"This is a historical moment, a decisive step for UTC. We now have an international reference time scale [that is] continuous for the next millennia." — Noël Dimarcq, via Nature
Georgette Macdonald, director general of the National Research Council of Canada's Metrology Research Centre in Halifax — Canada backed the resolution — put it this way: "This decision will ensure the continuity and relevance of UTC for centuries to come."
Since the leap second was introduced in 1972, timekeepers have added 27 of them — roughly one every year and a half — most recently at the end of 2016. After May 2027, there will never be a 28th.
What is a leap second, anyway?
The world runs on two different clocks, and they disagree.
The first is UTC, Coordinated Universal Time — the time your phone shows. It is kept by atomic clocks, which count the ultra-steady vibrations of caesium atoms (9,192,631,770 oscillations of a caesium-133 atom define one second). Atomic time never wavers.
The second clock is the Earth itself. Astronomers track UT1, the time measured by the planet's actual rotation — the time that puts the sun overhead at noon. And the Earth is a wobbly, unreliable timekeeper. The moon's gravity drags on the oceans and slowly brakes the planet's spin. Earthquakes shift mass around. The molten core sloshes against the mantle. Some days run a touch longer than 86,400 seconds; some run a touch shorter.
For most of history that didn't matter. But atomic clocks are so precise that the two time scales drift apart — by up to about a second every year or two. To keep civil time tethered to the turning Earth, timekeepers periodically inserted a leap second: a minute with 61 seconds, written as 23:59:60, usually at the end of June or December.
It sounds harmless. It was anything but.
The second that broke the internet
At midnight UTC on June 30, 2012, the world's clocks added one extra second — and a chunk of the internet fell over.
Reddit crashed, blaming "Java/Cassandra issues related to the leap second." LinkedIn went down. So did Yelp, Mozilla's services, Foursquare, Gawker and StumbleUpon. The culprit was a bug in the Linux kernel's timer code: when the clock repeated a second, some systems' CPUs spun into overdrive and locked up.
The chaos wasn't limited to websites. Qantas's passenger reservation system — built on the Amadeus platform used by dozens of airlines — crashed, delaying up to 50 flights and forcing staff to check in passengers by hand.
It happened again. The 2015 leap second prompted several Asian stock exchanges to halt or delay trading to avoid corrupted financial records — high-frequency trading systems timestamp transactions in microseconds and treat a repeated second as a potential catastrophe.
Google, burned by a leap second back in 2005, invented a workaround called the "leap smear": instead of inserting one jarring second, it stretches the extra second across a full 24 hours. Clever — but every company smeared differently, which created its own synchronization headaches.
By the 2020s, the tech industry had had enough. Google, Meta, Microsoft and Amazon all pushed timekeeping regulators to kill the leap second outright. As one former NIST physicist, Judah Levine, put it, the fix had "always been a problem." With AI data centers and ultra-low-latency trading now depending on flawless time, the risk of another one-second outage became unacceptable.
The nightmare scenario: the negative leap second
But the real reason the vote happened now — years ahead of schedule — is stranger and scarier.
In the mid-2010s, scientists noticed something odd: the Earth had started spinning slightly faster. If the speedup continued, timekeepers warned, they might soon have to do something never attempted in history — subtract a second. A negative leap second. Clocks would jump from 23:59:58 straight to 00:00:00.
No digital system has ever had to handle time running backwards. Engineers described the prospect as a "mini Y2K" — a sudden one-second rewind rippling through telecommunications, energy grids, transport networks, financial markets and satellite systems, with consequences nobody could fully predict.
"Avoiding negative leap seconds is a very big driver to open the window" between UTC and Earth-rotation time. — Nick Stamatakos, head of the Earth Orientation Department at the U.S. Naval Observatory, via New Scientist
Removing the leap second entirely takes that scenario off the table. There is no longer any mechanism that could force a backward jump — because from 2027, there is no mechanism that adjusts the clock at all.
Meet the leap hour
So what replaces the leap second? A far more patient idea: the leap hour.
Under the new rules, UTC is allowed to drift away from Earth-rotation time by up to a full hour — 3,600 seconds — before anyone has to do anything about it. Only when the gap approaches that size would timekeepers consider inserting a leap hour, a single one-hour correction.
Here's the catch: nobody expects to live to see it. The drift accumulates at a glacial pace — roughly a second or two per year. Scientists estimate it will take more than a thousand years, possibly several millennia, before a leap hour is needed. The resolution itself uses the cautious phrase "several centuries."
The vote doesn't even define how a leap hour would be inserted. There is no scheduled date and no agreed procedure — the resolution simply sets the maximum tolerance and asks the BIPM to keep measuring the gap. Astronomers, satellite operators and navigators who need Earth's exact orientation will keep receiving UT1 data as before.
The choice of an hour was deliberate. The timekeeping committee weighed three options: allowing a one-minute difference (good for about a century), a one-hour difference (good for roughly a millennium), or no limit at all. An hour won as the compromise between long-term stability and a defined engineering limit.
And honestly, we already live with bigger disconnects. Civil time in many places already differs substantially from local solar time because of time-zone politics, and daylight-saving shifts move clocks by a full hour twice a year in much of the world. Against that backdrop, a drift of a few seconds per century is invisible.
What else happened in Versailles
The leap second wasn't the meeting's only business. Delegates also asked for proposals on a new definition of the second itself — necessary because optical atomic clocks are now far more accurate than the caesium clocks that currently define it — and called on international agencies to start work on a universal time standard for the Moon.
As Dimarcq noted, the changes "will have a profound impact on future timekeeping both on Earth and other planets."
What does this mean for you?
Practically nothing — and that's the point. Your phone will keep working exactly as before. You will simply never again live through a 61-second minute. Behind the scenes, the software running banks, airlines, power grids and the internet gets one less apocalypse to plan for.
Your great-great-great-grandchildren's civilization — the one that finally has to schedule the first leap hour — might notice. You won't.
For now, the second hand ticks forward without interruption — probably for the rest of human history as we know it. The leap second is dead. Long live the leap hour.
What would you do with an extra hour — if you had a thousand years to plan for it? Tell us in the comments.
Frequently asked questions
Will my clocks jump forward an hour in 2027?
No. May 20, 2027 is simply the date the leap second ends — from then on, no extra seconds are added or removed. The gap between atomic time and Earth-rotation time will grow by only about a second a year, so a leap hour is centuries away. Nothing jumps.
What is the difference between UTC, UT1 and TAI?
UTC (Coordinated Universal Time) is the civil time standard your devices use — atomic time adjusted to stay near Earth-rotation time. UT1 is the astronomical time measured by Earth's actual spin. TAI (International Atomic Time) is pure, continuous atomic time with no leap adjustments at all.
Why was the leap second such a problem for computers?
Computers assume time always moves forward at a steady rate. A repeated second (23:59:60) breaks that assumption, causing bugs in operating systems, databases and trading platforms. In 2012 a leap second crashed Reddit, LinkedIn, Yelp and Qantas's booking system; later ones disrupted stock exchanges and cloud providers.
What is a negative leap second?
A second that would have been subtracted — rather than added — because Earth was spinning slightly faster than atomic time. It has never happened, and no software was designed to handle clocks jumping backwards, which is why engineers feared it as a "mini Y2K." The 2026 vote eliminated the possibility entirely.
When was the last leap second?
December 31, 2016. Twenty-seven leap seconds were added between 1972 and 2016 — roughly one every year and a half. After May 20, 2027, no more will ever be added.
Could the decision be reversed?
The resolution passed by majority vote, not unanimously, and the General Conference on Weights and Measures meets every four years — so in principle it could be revisited. But after three decades of debate, the direction of travel is clear.
Sources
- Nature — "World's timekeepers upend how they adjust time to avoid catastrophic 'negative second'" (Elizabeth Gibney, Oct 15, 2026)
- Scientific American — "Scientists vote to end leap seconds, redefining how the world keeps time" (Emma Gometz, Oct 15, 2026)
- New Scientist — "Scientists have just voted to abolish leap seconds" (Oct 16, 2026)
- Phys.org — "'Leap second' wreaks Internet havoc" (2012 outages)
- Network World — "Watch out for an upcoming leap second"
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