The Leap Second Dies Because Earth Sped Up

On 15 October 2026, the 28th General Conference on Weights and Measures voted to end leap seconds, locking in continuous UTC from 20 May 2027. Delegates meeting in Versailles raised the allowed gap between atomic civil time and Earth’s rotation from 0.9 seconds to one hour.

The tool was built because the planet was slowing. It dies because the planet sped up, and because a subtracted second has never been tested on the networks that now run the day.

CGPM Votes for Continuous UTC From May 2027

The conference, the treaty body behind the International Bureau of Weights and Measures, adopted Resolution 5 at its 28th meeting. BIPM will still produce Coordinated Universal Time. What changes is the tether to UT1, the scale that follows the Earth’s spin. continuous UTC on 20 May 2027 is the line the resolution writes down, and the maximum absolute difference |UT1-UTC| becomes 3,600 seconds.

Sixty-nine member states sit in that room. The UK’s National Physical Laboratory, writing on the day of the vote, said leap seconds had kept UTC within 0.9 seconds of UT1, and that from 20 May 2027 a difference of up to 1 hour will be allowed before any later intervention is needed. NPL also said the link is not cut: UT1 will still be measured, and the difference from UTC will still go out through the usual services.

OLD UTC AND NEW UTC

Noël Dimarcq, president of BIPM’s Consultative Committee for Time and Frequency, said before the vote, “After 30 years of debates, we are ready.” NIST, the US standards lab, put the same news in one line the next day: it has had its last leap second.

We have had our last leap second!

The General Conference on Weights and Measures (CGPM) voted to officially eliminate leap seconds, establishing continuous Coordinated Universal Time (UTC) effective May 20, 2027.

🔗 https://t.co/ED8xw4YOs3 pic.twitter.com/4fnup6PFKH

— National Institute of Standards and Technology (@NIST) October 16, 2026

A Negative Second That Software Never Tested

In November 2022 the same conference already decided that the maximum |UT1-UTC| would be increased in, or before, 2035. That was a plan to keep UTC continuous for at least a century. The 2026 vote does not invent the policy. It pulls the start date forward by eight years against that outer bound, and it writes the new limit as one hour.

Patrizia Tavella, director of BIPM’s time department, said the chance of a negative leap second is what convinced the group to set 2027. Industry tests, she added, “showed several failures with a negative leap.” Judah Levine, a fellow at NIST, put the same fear in plainer terms: positive leap seconds still cause trouble after 50 years, and a negative one would be worse because nobody has run it live.

Resolution 5 cites a March 2025 workshop run with the International Earth Rotation and Reference Systems Service. Experts there put the probability of a negative leap second at 30% by 2035. The text also says critical systems are largely unprepared, and that getting them ready could cost industries about as much as their millennium-bug work. Formal notes from ITU-T, URSI, IEEE 1588 groups, and companies asked BIPM to bring continuous UTC forward rather than wait out that risk.

The other fear in the resolution is quieter. If UTC kept stumbling, some operators would simply switch their clocks to a satellite time scale and stop treating BIPM’s second as the common reference. That is a standards problem dressed as a convenience.

Melting Ice Delayed a Negative Second

Leap seconds exist because a day is not 86,400 SI seconds long. Tides raised by the Moon bleed angular momentum out of the planet, so atomic clocks pull ahead of the stars. Since 1972 every correction has added a second, never taken one away. Around 2020 that pattern broke. Earth began to speed up, UT1-UTC turned upward, and a first subtraction stopped looking like a thought experiment.

The Core Turned the Planet Faster

Duncan Carr Agnew, a geophysicist at the Scripps Institution of Oceanography at the University of California San Diego, separated the pieces in a 27 March 2024 paper. After he stripped out the effect of polar ice, the liquid core’s slowing still showed up as a steady speedup of the rest of the Earth. Extrapolating those trends, he wrote, UTC as then defined would need a negative discontinuity by 2029.

If polar ice melting had not recently accelerated, this problem would occur 3 years earlier: global warming is already affecting global timekeeping.

Duncan Carr Agnew, Scripps Institution of Oceanography, Nature 628

Without the extra melt, the same forecast lands around 2026. The clocks are being spared a bug by ice that is already in the ocean.

Greenland, Antarctica and a Three-Year Delay

Meltwater from Greenland and Antarctica moves mass toward the equator, raising Earth’s moment of inertia, which slows the spin. That drag has been racing the core’s speedup. It did not reverse the speedup. It postponed the moment civil time would have had to skip a second.

Zinovy Malkin, watching UT1-UTC at Pulkovo’s Earth-orientation centre, now reads a further twist. The yearly average length of day, which went negative in 2020 as the planet sped up, has turned positive for the first time since June 2020 and is still rising. His October 2026 update says a negative leap second in the next few years looks unlikely on present predictions. That is a different window from the workshop’s 30% by 2035. Both can sit on the page. The vote did not wait for the near-term scare to be confirmed.

FROM THE FIRST LEAP TO THE LAST VOTE

1. 30 June 1972: The first leap second is inserted; TAI-UTC moves from its opening 10 seconds to 11.
2. 30 June 2012: A Linux kernel bug on the leap second knocks out airline booking software and several large websites.
3. 31 December 2016: The 27th and last leap second is inserted; TAI-UTC reaches 37 seconds.
4. 18 November 2022: The 27th CGPM decides the UT1-UTC cap will rise in or before 2035.
5. 27 March 2024: Agnew’s paper forecasts a negative leap by 2029, delayed three years by ice melt.
6. March 2025: A CCTF and IERS workshop puts the chance of a negative leap at 30% by 2035.
7. 6 July 2026: IERS Bulletin C 72 says no leap second will be added on 31 December 2026.
8. 15 October 2026: The 28th CGPM adopts Resolution 5.
9. 20 May 2027: Continuous UTC takes effect.

The irony is not that climate “saved” the internet. Ice bought a few years. Timekeepers spent those years deciding they would rather not own a 59-second minute at all.

Linux, Qantas and the 61-Second Minute

A positive leap second is a civil minute that runs to 61 seconds: 23:59:59, then 23:59:60, then midnight. Plenty of software has never seen that extra tick. On 30 June 2012 it did, and the bill arrived in public.

Amadeus, which runs the Altea reservation platform, said a Linux kernel bug triggered by the leap second took the system down. Qantas and Virgin Australia fell back to manual check-in. The same night, Reddit, LinkedIn, Mozilla, Yelp and Foursquare hit the same class of failure. Servers spun at 100% CPU on a timer livelock. Reboots and clock resets brought them back. The kernel patch was small. The outage was not.

THE 2012 FAILURES

• Airline bookings: Amadeus Altea dropped out after the 30 June 2012 leap, and Qantas and Virgin Australia processed passengers by hand.
• Public websites: Reddit, LinkedIn, Mozilla, Yelp and Foursquare all reported the same leap-second stall on Linux.
• The bug: The kernel livelocked in its high-resolution timers when the extra second arrived, so machines burned CPU without doing useful work.

Google stopped stepping its clocks in 2008. It stretches the extra second across a day so no process ever reads 23:59:60. Its public NTP docs now recommend a 24-hour linear smear from noon to noon UTC, a frequency shift of about 11.6 parts per million, inside the slack of most quartz crystals. Meta uses a smear as well, Levine notes. Bloomberg, among others, smears on a different window.

Those patches keep individual fleets alive. They also mean two servers can disagree about what time it is during the smear, and neither is ticking a true SI second while it runs. Resolution 5 says that split in so many words: the workarounds “do not conform to the SI unit.” Killing the leap second is, among other things, a way to stop asking every network on Earth to lie in a slightly different dialect.

Levine’s verdict after the vote was short. Life will have one less hassle.

Who Still Has to Track Earth’s Spin?

Phones, power grids and bank timestamps will keep using UTC. They will simply stop being told, twice a year at most, that a minute might have 61 seconds. The people who point telescopes, reduce very-long-baseline interferometry, and publish Earth-orientation parameters still need UT1. Their job does not get easier. The offset they apply, DUT1, is allowed to grow past one second and, in the end, toward one hour.

Tavella’s instruction to that group is blunt. Systems that rely on UTC have nothing extra to do. Systems that rely on UT1 have to keep ingesting UT1-UTC after that difference is larger than one second. IERS has already said it will change how it announces leap seconds to match the new resolutions. The data stream continues. The legal requirement that civil time stay inside 0.9 seconds of the sky does not.

The nice fact is that we have nothing to do!

Patrizia Tavella, director of the BIPM Time Department

Satellite navigation already split on this question. GPS and Galileo run continuous scales and ignore leap seconds. GLONASS was built to apply them, which is why Russia voted against the 2022 resolution and wanted the change held to 2040. The United Kingdom, once protective of Greenwich’s role in the history of the day, later dropped its objection. GNSS receiver makers now have a more ordinary problem: turn off any remaining leap-second reaction so a dormant code path cannot fire on a bulletin that will never come.

Radio time services get a longer fuse. ITU-R Resolution 655, from the 2023 World Radiocommunication Conference, lets stations keep the old DUT1 coding until 2035, and no later than 2040, where hardware cannot be replaced sooner. The sky’s offset will still be on the air. It will just be a bigger number.

Several Centuries Before Anyone Adds an Hour

Resolution 5 does not schedule a leap hour. It sets a ceiling of 3,600 seconds and says that ceiling should keep UTC continuous for several centuries. Tavella has said the group does not speak of a leap hour yet. Anyone who still needs Earth time is told to follow the published difference, not to wait for a one-hour jump that no living operator has to implement.

NPL’s note on vote day is the same idea in lab language: astronomy and celestial navigation still care about UT1, and that difference will continue to be disseminated. Civil time, meanwhile, is already allowed to sit more than an hour away from apparent solar time through time zones and daylight saving. The resolution uses that fact as cover. A future hour-scale step would be a political cousin of those shifts, not a new kind of physics.

The last leap second remains the one inserted on 31 December 2016. TAI is 37 seconds ahead of UTC and will stay there through the 20 May 2027 cutover, unless IERS surprises everyone before that date, which Bulletin C 72 has already declined to do for the end of 2026. After the cutover, minutes stay 60 seconds long. The planet can speed up or slow down without asking Linux for a 61st tick.

Clocks will look the same the morning after. The argument that ran for 30 years is over, and the side that wanted a continuous second won because the other side of the ledger, a subtracted second, was a test no one would fund. Earth sped up, ice bought a delay, and the people who still measure the spin will keep two times in their heads for as long as UTC and the sky refuse to match.

Frequently Asked Questions

What Is a Negative Leap Second?

A negative leap second is a civil minute with 59 seconds, so the clock would read 23:59:58 and then 00:00:00, skipping 23:59:59. No such step has ever been applied. Most leap-second code, including common POSIX behaviour that repeats 23:59:59 instead of exposing 23:59:60, was written on the assumption that extra seconds are only ever added.

Will a Leap Second Be Added in December 2026?

IERS Bulletin C 72, dated 6 July 2026, says no leap second will be introduced at the end of December 2026, so TAI-UTC remains 37 seconds through that date. The next historical slot after that would have been 30 June 2027, which falls after continuous UTC begins on 20 May 2027.

Do GPS Satellites Follow Leap Seconds?

GPS and Galileo keep continuous internal time scales and do not insert leap seconds, while GLONASS was designed to apply them, a mismatch that sat behind Russia’s 2022 vote against a 2035-or-sooner change. Users who need Earth-rotation time still apply UT1-UTC from navigation messages and IERS products rather than trusting a satellite clock to track the sky by itself.

How Will Radio Stations Send the Growing UT1 Gap?

ITU-R Resolution 655 (Rev. WRC-23) sets a transition so radiocommunication systems may keep disseminating the UT1-UTC difference in existing formats until 2035, and no later than 2040, where gear cannot be replaced earlier. Working Party 7A is studying how to code a difference larger than the old 0.9-second DUT1 field.

Why Does the Change Take Effect on 20 May 2027?

20 May is World Metrology Day and the anniversary of the Metre Convention signed in 1875, the treaty that created BIPM. Tavella said that date was chosen as the application day after the negative-leap tests, so the cutover lands on the bureau’s own calendar rather than on a June or December leap-second slot.