Timekeepers Vote to End the Leap Second in May 2027, Eight Years Ahead of Schedule

The world’s official clock will stop adding leap seconds on May 20, 2027. Member states at the General Conference on Weights and Measures, the treaty conference that oversees the international system of units, voted Thursday in Versailles to end the leap second and let clock time drift as far as an hour from Earth’s rotation.

Coordinated Universal Time, or UTC, is the time that computers, phones, planes and GPS run on, and it’s kept by atomic clocks. Earth’s spin is irregular, so since 1972 timekeepers have inserted an extra second whenever the planet fell close to a second behind. There have been 27 of them, and the last came in 2016, Scientific American reports.

Resolution 5, adopted Oct. 15 by majority vote, makes UTC continuous from next May and caps the gap between atomic time and Earth time at 3,600 seconds. Once that gap fills, the fix would be a leap hour, which scientists don’t expect to need for more than 1,000 years. The same conference had set a deadline of 2035 for this change in 2022. Thursday’s vote pulled it forward eight years.

Running the clock backward

Earth started spinning faster in the mid-2010s, which raised the odds of the first negative leap second, a second deleted from the clock instead of added. Clocks would jump from 23:59:58 straight to midnight, as Meta’s engineers described it in a 2022 post. One has never been applied.

The resolution cites a March 2025 workshop, which included experts on Earth’s rotation, that put the chance of a negative leap second at 30% by 2035. Getting ready for one, it says, would cost the industries that depend on synchronized time an amount “estimated to be similar to their preparations for the millennium bug.” Patrizia Tavella, who runs the time department at the International Bureau of Weights and Measures, told Wired that tests by an industry standards group “showed several failures with a negative leap.”

The biggest clocks had already gone their own way

Positive leap seconds still cause “continuing problems” after 50 years, Judah Levine of the National Institute of Standards and Technology told Wired. A 2012 one scrambled Qantas’s booking system. Levine said Google and Meta “smear” each one instead, running their server clocks a hair slow for hours until the extra second is absorbed. Meta wrote in 2022 that it spreads the second over 17 hours and backed an industry push to freeze the count at 27.

The resolution says those smearing methods, used differently by different “global service providers,” leave systems out of step with each other and keep time that does “not conform to the SI unit,” the official definition of a second. Standards groups, including the ITU’s telecom arm and the IEEE working group behind precision network timing, filed formal statements asking for the change sooner. And the resolution warns that a delay “could cause some user organizations to change their reference time scales to, for example, a GNSS time instead of UTC.” GNSS covers the satellite navigation systems, GPS among them, that broadcast their own time.

With the biggest server operators already running a second of their own and satellite time named as the next exit, moving the end date up to 2027 is how the timekeepers keep users on UTC.

Who still needs the sun

“If systems rely on UTC, they have nothing to do,” Tavella told Wired. Systems that need time by Earth’s rotation will read the growing difference from satellite navigation messages or from the International Earth Rotation and Reference Systems Service, the body that tracks the planet’s spin, the resolution says.

To argue an hour of drift is tolerable, the resolution points out that time zones already put many countries more than an hour off solar time, and daylight saving moves clocks a full hour as a matter of routine.

The vote trades a 30% chance of a negative leap second by 2035, plus Y2K-sized costs to get ready for it, for a leap hour more than 1,000 years out, a bill addressed to nobody now alive.

Sources

• BIPM: CGPM Resolution 5 (2026), On the technical actions needed to ensure the continuity of UTC (adopted Oct 15, 2026)
• BIPM: Statement by the CCTF President, 28th CGPM, Versailles (Oct 15, 2026)
• Scientific American: Scientists vote to end leap seconds, redefining how the world keeps time (Emma Gometz, Oct 15, 2026)
• Wired: Pour One Out for the Leap Second (Brian Kahn, Oct 16, 2026)
• Engineering at Meta: It’s time to leave the leap second in the past (Oleg Obleukhov, Ahmad Byagowi, July 25, 2022)