Delegates in Versailles vote to end the leap second from May 2027
The General Conference on Weights and Measures voted in Versailles to end the leap second, with UTC going continuous on May 20, 2027. UTC may then drift up to 3,600 seconds from Earth's rotation before a leap hour is added.
The Scientist · Science desk
Key figures: the allowed difference between UTC and UT1 is extended to 3,600 seconds under the leap hour, against within one second under the leap second. Scientists predict a leap hour will not be needed for more than 1,000 years, a disputed prediction.
| Figure | Value | Note | Claim |
|---|---|---|---|
| Allowed UTC–UT1 difference with a leap hour | 3,600 seconds | 5 | |
| Leap-second UTC–UT1 limit | 1 second, within | 12 | |
| Time until a leap hour is needed, per scientists | 1,000 years, over | disputed | 15 |
What happened
- The first leap second went into UTC in 1972 at the request of astronomers and other experts, and 27 have been added since.
- Earth's spin began speeding up in the mid-2010s, and scientists predicted a negative leap second might be needed as early as 2029.
- The resolution passed by majority vote at the CGPM, the conference the BIPM hosts every four years for its member states.
- Since 2022, scientists, government officials and telecommunications professionals had been urgently searching for a practical fix.
Why it matters
- What's now possible capability From May 2027 UTC stops stepping, which removes a recurring discontinuity that digital systems had to survive.
- What it limits constraint UTC no longer stays within a second of Earth's rotation. Anyone who read rotation off UTC now gets an answer 3,600 times looser.
- Who has to decide decision The leap hour is forecast to be more than 1,000 years away, so the planning date for operators is May 20, 2027.
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BIPM's 69 member states vote this week on retiring the leap secondConfirmed2 publishers
UTC is a weighted average of atomic clocks around the world, disseminated by the International Bureau of Weights and Measures [11]. Earth's rotation is irregular, so a UTC day is close to a UT1 day but not identical to it [12]. The leap second kept the two within one second of each other [12]. From May 20, 2027, the allowed gap is an hour [1][5]. That is 3,600 times the old tolerance [13].
For operators, the old problem was the step itself. UTC is the reference for computers, phones, boats, planes, satellites and GPS [11]. Scientific American reports that switching to continuous time will be easier for digital infrastructure, with much less risk of bugs or service outages from a leap second discontinuity [9].
The negative case was the sharper worry. Engineers say a negative leap second would be like a "mini Y2K" because it would suddenly send the entire world one second backward in time [3]. The earliest forecast for one, 2029, falls two years after the new effective date [14]. "Avoiding negative leap seconds is a very big driver to open the window," said Nick Stamatakos, who runs the Earth Orientation Department at the U.S. Naval Observatory, the agency in charge of the nation's GPS grid [4][7].
The leap hour is the backstop, and scientists predict it will not be needed for more than 1,000 years [15]. We think no system in service today needs to be designed around a leap hour, provided that forecast holds. The date to plan around is 2027.
The losers from the wider gap are users of the sky. The leap second was first added at the behest of astronomers and other experts [16], and Scientific American does not list which systems rely on the one-second bound or how users who need Earth's rotation will get it once UTC stops tracking it.
"After 30 years of debates, we are ready," said Noel Dimarcq, a senior scientist at the Centre National de la Recherche Scientifique, just before the vote [8].
What to watch
- How users who need Earth's rotation angle will obtain UT1 once UTC no longer tracks it within a second.
- Whether Earth's spin keeps speeding up, since that was the trigger for the 2029 negative leap second forecast.
- How the first continuous UTC behaves in infrastructure after May 20, 2027.
Clarity's read
What the record supports and how the coverage leans. The claims behind it follow.
Reality
- Evidence78
- Adoption40
- Hype gap+12
- Incentives40
- Confidence78
Perspective Coverage
7 publishers- Builder
- Builder 31%
- Operator
- Operator 51%
- Investor
- Investor 18%
Claim ledger
Ranked by verification strength, evidence, and original report placement.
- [1]
Officials at the General Conference on Weights and Measures (CGPM) in Versailles, France voted to end the leap second and make UTC continuous, with the change taking effect on May 20 (World Metrology Day), 2027.
- [2]
The resolution to end the leap second won by a majority vote at the CGPM, which the International Bureau of Weights and Measures (BIPM) hosts every four years to discuss issues with member states.
- [3]
Engineers say a negative leap second would be like a 'mini Y2K' event because it would suddenly send the entire world one second backward in time, which could cause chaos in digital infrastructure.
- [4]
"Avoiding negative leap seconds is a very big driver to open the window," said Nick Stamatakos, head of the Earth Orientation Department at the U.S. Naval Observatory.
- [5]
The leap second will be replaced with a leap hour, added much more infrequently, which extends the allowed difference between UT1 and UTC to an hour, or 3,600 seconds.
- [6]
Earth's spin began speeding up in the mid-2010s, and scientists predicted a negative leap second would be necessary soon, perhaps as early as 2029.
- [7]
The U.S. Naval Observatory manages the United States' GPS grid.
- [8]
"After 30 years of debates, we are ready," said Noel Dimarcq, a senior scientist at the Centre National de la Recherche Scientifique (CNRS), just before the vote.
- [9]
Switching to continuous time will be easier for digital infrastructure, with much less risk of bugs or service outages arising from a leap second discontinuity; the change won't be noticed by most people.
- [10]
Since 2022 scientists, government officials, telecommunications professionals and others have been urgently trying to find a practical way to solve the leap second problem.
- [11]
UTC is the world's standard timescale and the reference for how computers, phones, boats, planes, satellites and other systems tell time and for how navigation systems such as GPS function; it is calculated via a weighted average of atomic clocks around the world and disseminated by the BIPM.
- [12]
A UTC day is close to a UT1 day but not exactly the same because Earth's rotation is irregular; the leap second has kept UTC aligned with UT1 within one second of difference.
- [13]
The new allowed UTC-UT1 difference of 3,600 seconds is 3,600 times the old one-second tolerance.
- [14]
The earliest predicted negative leap second (2029) falls two years after the May 20, 2027 effective date of continuous UTC.
- [15]
Scientists predict a leap hour will not be needed for more than 1,000 years.
- [16]
The first leap second was added to UTC in 1972 at the behest of astronomers and other experts to realign the time standard with Earth's rotation; since 1972, 27 leap seconds have been added.
Sources
7 independent publishers whose own reporting we read for this story.
- interestingengineering.comEarth could see a 25-hour day in 1,000 years as scientists scrap the leap second
1 article · October 16, 2026
- nature.comWorld's timekeepers upend how they adjust time to avoid catastrophic 'negative second'
1 article · October 14, 2026
- newscientist.comScientists have just voted to abolish leap seconds
1 article · October 16, 2026
- scientificamerican.comScientists vote to end leap seconds, redefining how the world keeps time
1 article · October 15, 2026
- theguardian.comHow an accelerating Earth led experts to adopt the leap hour
2 articles · October 15, 2026
- theverge.comTime is a-changin’: say goodbye to the leap second
1 article · October 16, 2026
- wired.comThe Leap Second Is No More
1 article · October 16, 2026
Topics and entities
Topics
- Digital infrastructure reliability
- Timekeeping standards
Entities
- Coordinated Universal Time
- International Bureau of Weights and Measures
- General Conference on Weights and Measures
- U.S. Naval Observatory
- National Physical Laboratory
- Patrizia Tavella