A vote in Versailles sets a new date

The General Conference on Weights and Measures (CGPM), meeting in Versailles, voted on Thursday, 15 October 2026, to stop inserting leap seconds into Coordinated Universal Time (UTC). The change takes effect on 20 May 2027. The CGPM capped the permitted gap between UTC and the Earth-rotation scale UT1 at 3,600 seconds, or one hour. Its 28th meeting also asked the International Bureau of Weights and Measures (BIPM) to keep working with the ITU, the International Astronomical Union and other affected bodies so that users are informed. Japan's economy ministry took part in the conference, Jiji Press reports, and Japan's National Institute of Advanced Industrial Science and Technology counts 27 leap seconds added since 1972.

Why the change came early

The 2022 decision had set 2035 as the cut-off, so the new date arrives eight years early, according to Berliner Zeitung. The reason is that Earth is turning faster, which could make a negative leap second necessary for the first time. Such a step would move clocks from 23:59:58 straight to midnight, and no one has tried it. Duncan Agnew, a US geophysicist, reported in Nature in 2024 that meltwater from Greenland and Antarctica drifts toward the equator, widening the planet and slowing its spin, which delayed that negative second. Without this effect, the negative second would have been due in 2026. Thomas Grenon, director general of the Laboratoire national de métrologie et d'essais (LNE), noted that the time people live by follows Earth's rotation, which varies with tides, the Moon, the Sun and fluid movements in the planet's core.

Software risks and past glitches

Leap seconds are hard for computer systems, because few pieces of software expect a 61st second, Berliner Zeitung reports. On 1 July 2012, a Linux bug triggered by an extra second froze the Amadeus booking system, forcing Qantas to check in passengers by hand while some flights ran up to two hours late. El País gives a different account of the Qantas problems, saying a badly introduced adjustment caused billing and scheduling errors on more than 400 flights that year. The Verge lists outages at Foursquare, Reddit, LinkedIn and Yelp during earlier leap seconds, and says Meta, Google, Microsoft and Amazon backed scrapping the system. Setnam Shemar, principal scientist at the National Physical Laboratory, said the decision lowers the risk to systems that depend on precise time:

“The decision taken today reduces the risk to systems that depend on precise and continuous time synchronisation, including national infrastructure that we rely on daily.”

What changes next

From 20 May 2027 the clock runs without extra seconds, and the gap with astronomical time will widen at an irregular pace because Earth's rotation is hard to predict, El País reports. A correction will come only when the gap nears one hour. The Verge puts that point about 3,000 years away, while Europa Press cites the resolution saying UTC will stay continuous for several centuries. Shemar explained why the link between the two scales still matters:

“The relationship between UTC and UT1 is important for some applications, such as astronomy and celestial navigation.”

Phones, financial markets and power grids keep relying on UTC, which is produced from atomic clocks such as those at the National Physical Laboratory in the UK. The BIPM will regularly report to the CIPM on how far UTC and UT1 have drifted apart.