World timekeepers vote to stop adding leap seconds from 20 May 2027

At the end of 31 December 2016, the world’s official clocks showed a time most of us never see: 23:59:60. That extra second, the 27th since 1972, may now be the last. In a leap second vote on Thursday 15 October 2026 in Versailles, near Paris, the world’s timekeepers agreed to stop adding them from 20 May 2027.

The decision is less about the sky than about computers. Leap seconds keep world clock time, UTC (Coordinated Universal Time), within one second of Earth’s spin. But each one jolts the networks behind banks, phones, power grids and satellites. The vote ends 30 years of debate, said Noël Dimarcq, who leads the time committee of the International Bureau of Weights and Measures (BIPM).

What the leap second vote changes on 20 May 2027

The General Conference on Weights and Measures (CGPM), where the BIPM’s 69 member states vote, approved Resolution 5 by a majority; no count was published. The start date is World Metrology Day.

From then on, UTC runs without jumps. Its gap from Earth’s rotation time can grow to one hour, instead of under one second today. The BIPM’s English text says that limit keeps UTC continuous “for several centuries”. Other estimates run longer: Patrizia Tavella, who directs the BIPM’s time department, told CNN more than 3,000 years. Nobody can say exactly, because Earth’s spin is unpredictable.

So, are leap seconds abolished? In effect, yes. A 2022 meeting had set the change for “in, or before, 2035”; this vote brings it forward.

The second nobody has ever removed

Over the long term, Earth’s spin is slowing, but in recent years it has sped up. Changes such as melting polar ice also shift it, CNN noted. So the next correction might have to remove a second. That is a negative leap second: clocks would skip 23:59:59. It has never been done.

The resolution cites a March 2025 expert workshop that put the chance of one at 30% by 2035. The BIPM says preparing time-dependent industries for it would cost about as much as their Y2K (millennium bug) work. Even the usual extra second caused trouble: in 2012 it hit Reddit, LinkedIn and Qantas’s booking system. Big tech firms also spread it out in their own ways, so systems quietly disagreed about the time.

Critics say big tech gains, and the hardest choice waits

Marek Ziebart, a space geodesy expert (geodesy measures Earth’s shape and spin) at University College London, argued before the vote that the technical problems are overstated and that tying time to Earth’s rotation has value. Michelle Bastian of the University of Edinburgh wrote in The Conversation that the outcome favours big tech over astronomers and other users.

Russia opposed the change in 2022, because its GLONASS satellite system builds leap seconds in; how it voted this time is not confirmed. Tavella told timeanddate.com that no solution pleases everyone.

Despite some headlines, the resolution never mentions a “leap hour”. It only sets the one-hour limit and leaves the rest to a future generation. “It postpones the decision,” Christian Bizouard of the International Earth Rotation and Reference Systems Service (IERS) told timeanddate.com.

Your phone will not notice, but astronomers and radio clocks will

For most readers, nothing changes. Phones and watches take their time from networks, so there is nothing to install. Time zones and daylight saving stay as they are, and clock time will drift from sun time too slowly to feel.

Specialists feel it instead. Astronomers and navigators who need Earth’s true rotation will use published corrections from satellite signals and IERS. Radio time signals may take until 2035, and no later than 2040, to adapt. If you build software for trading or networks, now is a good moment to check how it handles time.

A sensible trade that leaves one hard question for later

On balance, this looks like a careful decision. It removes an untested risk and keeps one shared world clock, where delay, the resolution warns, could have pushed some users to satellite-navigation time instead. Telecoms and engineering bodies asked for it sooner.

The costs are real too. Our clocks loosen an old link to the sky, and the critics have a point that big networks gain the most. The hardest question, what to do near one hour, is passed on. Still, the likelier reading is that a small, steady drift is safer than sudden jumps nobody has practised.

When was the last leap second? The answer, 31 December 2016, now looks set to hold for good: the IERS has already ruled one out at the end of 2026, the last chance before the switch. Watch how Russia’s GLONASS adapts; we will follow it in our science news. After 20 May, the world’s clocks simply keep counting.