The world's timekeepers vote to end the leap second in 2027

You live by two clocks at once, even if you never notice. One is atomic time, which ticks at an almost perfectly steady rate. The other is Earth itself, which spins a little faster or slower from year to year.

Coordinated Universal Time (UTC) sits between them. It's the time standard behind your phone, your laptop and most countries' civil time. To keep it close to Earth's spin, timekeepers have added an extra second now and then.

Scientific American reports that the leap second was introduced in 1972 by the International Bureau of Weights and Measures (BIPM). CNET counts 27 leap seconds added since then. The last one came in 2016.

Think of it like a band playing along with a dancer who keeps changing speed. Every few years, the band holds one note a little longer so they land on the same beat again.

The decision came from the General Conference on Weights and Measures (CGPM), the BIPM's policy-making body. Its member states met at the Palais des Congrès in Versailles, France, from October 13 to 15, 2026. CNET reports the vote came on Thursday.

The key text is Resolution 5, "On the technical actions needed to ensure the continuity of UTC." Here's what it sets:

• Start date: continuous UTC becomes effective on May 20, 2027.
• New limit: the gap between UTC and Earth's rotation may grow to 3,600 seconds, or 1 hour.
• How long that lasts: the CGPM says this keeps UTC continuous "for several centuries."
• What's next: the BIPM will keep working with the International Telecommunication Union (ITU), astronomers and the International Earth Rotation and Reference Systems Service (IERS) to inform users and keep watching the gap.

From May 20, 2027, no leap seconds will be added to world time, positive or negative.

Computers need time to move forward in a clean, steady line. They use timestamps to put messages, payments and records in the right order. A clock that suddenly shows 23:59:60 can confuse software that never expected that second to exist.

By the numbers: CNET reports that the leap seconds of 2012, 2015 and 2016 caused trouble for some computer systems. The worst was 2012, when the extra second crashed websites and software including Reddit, LinkedIn and Mozilla.

Scientific American adds that leap seconds have caused outages at Meta, Reddit and Cloudflare, grounded planes and affected high-frequency stock trading. Judah Levine, a former physicist in the Time and Frequency Division at the US National Institute of Standards and Technology (NIST), told the magazine the fix has "always been a problem."

In real life Imagine your bank's computer records your paycheck at 23:59:60 and your rent payment at 23:59:59. If its software can't handle that extra second, it might think the rent left before the money arrived.

Some big tech companies built their own workaround. Smithsonian Magazine, as summarized by Slashdot, reports that companies such as Google and Meta "smear" the leap second: instead of adding it all at once, they spread it out in tiny pieces over a longer stretch. You can read more about Meta's tech on our Meta topic page.

It works, but it creates a new issue. The CGPM's resolution says the different methods used by global service providers leave timing slightly inconsistent between systems. Those smeared seconds also don't match the official unit of time.

The resolution also warns of a bigger risk. If fixing UTC took too long, some organizations might switch to satellite navigation time instead of UTC. That would weaken the whole idea of one shared world time.

This is where it sounds like a season finale. Earth started speeding up.

Scientific American reports that Earth's rotation began speeding up in 2016, which is why no leap seconds have been added since. If that continues, timekeepers would need the first ever negative leap second: one second removed from the clock instead of added.

A workshop held by the BIPM's time committee and the IERS in March 2025 estimated that the chance of a negative leap second would rise quickly, reaching 30% by 2035, according to the resolution. The CGPM says a negative leap second "has not previously been applied" and poses a high risk of disruption to critical infrastructure that is largely unprepared.

The catch: getting ready would cost real money. The resolution estimates the investment for time-dependent industries would be similar to their preparations for the millennium bug, better known as Y2K.

Patrizia Tavella, director of the BIPM's Time Department, explained the urgency to Scientific American before the vote. "We estimated that if we wait till 2035, we have 30 percent risk of a negative leap second," she said. When the BIPM asked users and other organizations, they said, "No, even 10 percent risk is too much."

"It's never happened before, and therefore the software for that has never been tested."

That's Darryl Veitch, who leads the Timing Lab at the University of Technology Sydney, as quoted by the Australian Broadcasting Corporation and cited by Smithsonian Magazine.

Plenty of groups. The resolution says that transportation, energy distribution, telecommunication and space projects need UTC to run "without any future adjustment."

It lists formal statements to the BIPM from the ITU's telecom standards sector, the International Union of Radio Science, the IEEE 1588 working group on network clock synchronization, and individual industry representatives. All of them asked to bring the change forward.

The CGPM also points to the growing number of connected devices, smart power grids and online services. More connected systems means more places where one odd second can break something.

This decision builds on a vote from 2022. Back then, the CGPM decided to raise the allowed gap between UTC and Earth's rotation "in, or before, 2035." The new resolution picks the exact date and the exact limit, eight years early.

Yes, but very, very slowly. CNET notes that in the future, the sun might not be at its highest point exactly at noon. Since only 27 leap seconds were needed in more than 50 years, any change in your lifetime will be tiny.

The CGPM makes a practical point too. In many countries, local sun time already differs from clock time by more than an hour because of time zones. And daylight saving time moves clocks by a full hour twice a year in many places.

Is a "leap hour" coming one day? Christian Bizouard, director of the IERS Earth Orientation Center, told The Guardian, as cited by Smithsonian Magazine: "If a leap hour happens, it won't be for a very, very long time. What we are doing is pushing back the decision."

People who still need Earth's exact rotation, like astronomers, can get the current difference from satellite navigation messages and online services from the IERS and national metrology institutes, the resolution says. Some radio time signals get a transition period until 2035, and no later than 2040, under an ITU rule.

• Your devices: nothing changes on your screen. Your phone and laptop keep showing the same time.
• Fewer glitches: the rare leap second nights that knocked out websites and apps are on their way out after May 20, 2027.
• No Y2K sequel: the scary first negative leap second most likely never happens now.
• Noon stays noon: the drift from the sun is so slow you won't notice it.