Nations vote to replace leap second with leap hour in 2027
- Delegates from 69 countries voted to replace the leap second with a leap hour.
- The change will take effect on May 20, 2027, to protect global infrastructure.
- Earth has been accelerating since 2020, increasing the risk of a negative leap second.
- Negative leap seconds pose significant risks to software that has never been tested for them.
- The new continuous UTC system allows time to drift by up to an hour before adjustment.
Delegates from 69 countries voted at a conference of the International Bureau of Weights and Measures in Versailles, France, on Thursday 15 October 2026 to replace the leap second with a leap hour starting in 2027. A proposed resolution seeks to abandon the leap second and potentially replace it with a leap hour as early as 2027.
Earth's acceleration creates 30% risk of negative leap second
Earth has been accelerating since 2020, creating a 30% risk of needing a negative leap second to remove a second from the clock. Patrizia Tavella, director of the Time Department at the Bureau, noted that waiting until 2035 would maintain this 30% risk. Global infrastructure operators consider even a 10% risk of a negative leap second to be unacceptable. A negative leap second would force clocks to skip a second, running from 23:59:57 to 00:00:00, which would effectively erase two timestamps. While the leap year was introduced over 2,000 years ago to keep the calendar year in step with Earth's orbit, the new leap hour resolution aims to provide a buffer that could last for centuries, effectively decoupling civil time from the rotation of the planet.
Technical infrastructure faces risk from clock adjustments
The leap hour will be adopted starting next year to reduce the risk of technical systems and computers failing when processing minutes that are 61 or 59 seconds long. Following the 2012 leap second, computer systems including Reddit and LinkedIn crashed and Qantas flights were delayed because operating systems could not cope with a 61-second minute. Dr Setnam Shemar, principal scientist at the National Physical Laboratory, warned that negative leap seconds pose even greater risks because software has never been tested for them. Positive leap seconds have previously caused significant outages for major tech companies including Meta and Cloudflare, as well as airline reservation systems and high-frequency trading platforms. Major cloud providers use proprietary leap second smearing to spread the extra second across a 24-hour window, but these algorithms are not standardized across platforms.
Experts weigh in on planetary volatility
Dr Setnam Shemar, principal scientist in the time and frequency department at the UK’s National Physical Laboratory, stated that the leap hour will reduce risks to systems and national infrastructure while preserving the link between the global time system and Earth's rotation. Dr Shemar noted, "After 30 years of debate, the leap second is making way for the leap hour". Duncan Agnew, a professor emeritus of geophysics at UC San Diego, described planetary acceleration as inherently volatile. Leonid Zotov of Moscow State University suggested the spin might naturally slow again. Graham Jones, an astrophysicist at the Time and Date website, stated that the threat of a negative leap second helped focus the minds of experts regarding the change. Claire Nichols from Oxford University noted that the solar system contained more energy 4.5 billion years ago, causing the Earth to rotate rapidly, and that the Tyrannosaurus Rex had half-an-hour less time in a day than humans do now. In 1955, Louis Essen and Jack Parry at the National Physical Laboratory built the first practical, high-precision caesium atomic clock, and in 1967, the formal definition of a second was set to 9,192,631,770 vibrations of the caesium atom. While UTC is distributed via the Network Time Protocol, GPS satellites run on atomic time and ignore leap seconds by broadcasting a static offset.
Historical context of leap seconds and Earth's rotation
Since 1972, 27 leap seconds have been added to ensure UTC remains aligned with astronomical time. Earth's rotation is generally slowing down over the long term, with a day lasting about 23 and a half hours when an asteroid doomed the dinosaurs. This long-term slowing is due to lunar tidal friction, a phenomenon documented in historical records of solar eclipses dating back over 3,000 years.
Natural forces driving current rotation changes
Earth set a new rotation speed record on July 4, 2024, completing its daily spin 1.66 milliseconds faster than the standard 86,400 seconds, a record that was further challenged throughout 2025. The recent acceleration is primarily attributed to changes in the planet's liquid metal core. Earth's rotation is influenced by natural forces including the molten core, global jet streams, and the shift of the oceans. Since roughly 2020, the spin has accelerated due to factors including shifts in the liquid outer core, changes in ocean currents, and the redistribution of mass linked to melting polar ice caps. The IERS Earth Orientation Centre in Paris tracks changes in Earth's rotation and issues notices when leap seconds are required. Dr Christian Bizouard is the director of the IERS Earth Orientation Centre in Paris.
Continuous UTC implementation scheduled for May 2027
The new system, called continuous UTC, will come into effect on May 20, 2027.
Frequently Asked Questions
- How an accelerating Earth led experts to adopt the leap hour | Science | The Guardian (theguardian.com)
- How an accelerating Earth led experts to adopt the leap hour | Science - United States News Beep | NewsBeep.com (newsbeep.com)
- Global Timekeepers Weigh Shift to ‘Leap Hour’ to Avoid Catastrophic Negative Leap Second (azat.tv)
- BIPM Delegates Vote on Proposal to Abolish the Leap Second by May 2027 - Newsy Today (newsy-today.com)
- World timekeepers approve leap hour to reduce risks to computer systems (bdnews24.com)
- The sun will no longer dictate the length of a day (telegraph.co.uk)
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