In 2022, international timekeepers voted to phase out the leap second—an extra tick of time added to account for Earth’s erratic rotation. But as our planet’s spin grows weirdly faster, timekeepers are seriously considering introducing a larger fix: the leap hour.
Every four years, scientists and timekeeping authorities convene at the General Conference on Weights and Measures (CGPM) to discuss relevant news about standards for maintaining the Coordinated Universal Time (UTC). After abandoning leap seconds in 2022, the conference resolved to decide on an alternative with a higher time limit by 2035. However, due to recent upticks in Earth’s rotation rate, timekeepers are having to reckon with negative leap seconds—the consequences of which are prompting authorities to find a solution before the 2035 deadline.
And one viable solution is to replace the leap second with the leap hour. Timekeepers will vote on this idea during the next CGPM, which will be held in October this year.
“We estimated that if we wait till 2035, we have 30 percent risk of a negative leap second,” Patrizia Tavella, director of the time department at the International Bureau of Weights and Measures (BIPM), told Scientific American.
Why the hurry?
A full day on Earth—the time it takes for our planet to complete one rotation on its axis—lasts approximately 86,400 seconds. But factors such as the Sun’s position or the Moon’s orbits can create small variations, which humanity is much more privy to thanks to advances in scientific technology. In 1972, the BIPM introduced a leap second to account for the tiny discrepancies between UTC and UT1, or astronomical time.
To most of us, a difference of a couple milliseconds probably seems insignificant. But to people working with high-precision systems in communications, GPS, banking, etc., even the tiniest error margins can mess up entire networks. For instance, a leap-second-related bug caused failures in Cloudflare’s DNS service when a leap second was added to UTC at midnight on January 1, 2017.
In other words, ironically, leap seconds were created to fix time discrepancies but ended up creating a “technical nightmare” at odds with modern society’s highly computerized infrastructure, according to a report on the 2022 resolution by The New York Times.
Entering the negative
In that sense, some experts believed that the leap second has “always been a problem,” as Judah Levine, a former physicist for the time division at the U.S. National Institute of Standards and Technology, told Scientific American. But the problem became more potent as, starting in 2016, Earth started to spin unusually fast.
For instance, on July 4 of 2024, Earth set a new record by completing its daily spin 1.66 milliseconds faster than usual. Around one year later on July 10, 2025, it spun 1.36 milliseconds faster than usual, with other shorter-than-usual days occurring on July 9 and July 22. This has led experts to warn about a “negative” leap second—subtracting a second from UTC—to keep UTC aligned with UT1 as early as 2029.
Given the issues with positive leap seconds, it’s likely that negative leap seconds won’t be any less problematic, as the CGPM noted in its 2022 resolution. A larger margin in the form of a leap hour might soften the stress on our computational systems, as the additions would happen much less frequently and stakeholders would have more time to prepare for the leap hour, according to the CGPM resolution.
To Scientific American, Tavella said she believes there is “sufficient urgency” to implement the leap hour as soon as possible. “We went to our users, stakeholders and other organizations and said, ‘What do you think? 30 percent risk of a negative leap second is an issue, or you can accept [it]?’” Tavella recalled. “And they said, ‘No, even 10 percent risk is too much.’”
