The leap second is on its way out: world timekeepers move to drop it from May 2027
Meeting in Versailles, the world's metrology body considered ending leap seconds eight years early, to avoid an unprecedented 'negative' second as Earth's spin speeds up.
At a glance
- Delegates at the 28th General Conference on Weights and Measures in Versailles voted this week on a plan to stop adding leap seconds from May 20, 2027.
- The 2022 conference had already agreed to phase them out by 2035; the new date brings that forward.
- Earth's rotation has sped up, raising the prospect of the first-ever negative leap second — something computer systems are not ready for.
- UTC would be allowed to drift up to an hour from Earth-based time before any correction.
The world's clocks are about to get simpler. Delegates at the 28th General Conference on Weights and Measures (CGPM), meeting this week in Versailles, considered a resolution to stop adding leap seconds to Coordinated Universal Time from May 20, 2027 — eight years earlier than previously planned. Euronews reported on October 16 that world time is set to drop the leap second from May 2027.
What a leap second does
Atomic clocks keep near-perfect time; the Earth does not. Its rotation varies slightly, so since 1972 timekeepers have added a "leap second" 27 times to keep UTC aligned with the planet's spin. The last one was added at the end of 2016.
Why it became a problem
Leap seconds arrive irregularly with only six months' notice, and software often struggles with a 61-second minute. A leap second in 2012 disrupted Reddit, LinkedIn and Qantas's booking system, and the 2016 one caused brief problems for Cloudflare customers. Meta's engineers have called the leap second a major source of pain, and companies such as Google "smear" the extra second across several hours to avoid crashes.
The negative-second danger
The bigger worry is new. Earth's rotation has recently sped up, which could require subtracting a second for the first time — a jump from 23:59:58 straight to midnight. No critical system has ever had to handle that. Estimates from the International Earth Rotation and Reference Systems Service put the chance of needing a negative leap second by 2035 at around 30 percent.
From leap seconds to a 'leap hour'
Under the proposal, UTC would be allowed to drift up to an hour from astronomical time before any correction, which experts say keeps UTC continuous for several centuries. Earth-rotation time will still be measured and published. The 2022 conference had already voted to phase out leap seconds by 2035; some countries, including Russia and the UK, have opposed the change in the past.
For most people nothing will change. For the engineers who run the internet, it removes one of timekeeping's oddest hazards.
Questions people are asking
What is a leap second?
An extra second occasionally added to Coordinated Universal Time (UTC) to keep atomic clocks in step with Earth's slightly irregular rotation. It has been used 27 times since 1972, most recently at the end of 2016.
Will I notice the change?
Almost certainly not. Clocks will simply stop receiving occasional one-second corrections. The main beneficiaries are IT systems, networks and financial markets, which have suffered outages around leap seconds.
How we verified this story
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