International timekeeping authorities are preparing to vote on a proposal to make the leap second—an extra second that is occasionally added to the year to keep Coordinated Universal Time (UTC) in tune with Earth’s rotation—into a leap hour as early as 2027.
The leap second was introduced in 1972 by the International Bureau of Weights and Measures (BIPM) to match UTC with Earth’s gradually slowing spin. Every time UTC becomes one second too fast for Earth, timekeepers add a leap second to UTC.
This fix has “always been a problem,” says Judah Levine, a former physicist for the Time and Frequency Division at the U.S. National Institute of Standards and Technology.
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It particularly affects computer systems, which require precise time to send and receive messages in the correct order. Adding a second to the clock has caused Meta, Reddit and Cloudflare outages, grounded planes, and affected high-frequency stock trading. As digital networks demand faster processing speeds, some developers have ignored the BIPM’s leap second standard and instead implemented their own fixes, such as “smear seconds” that jive better with computers.
In 2016 Earth’s rotation started speeding up, so leap seconds have not been added since then. The accelerating rotation is fast enough, however, that timekeepers foresee the need for the first ever negative leap second—which could cause unknown and potentially catastrophic infrastructure problems.
Timekeepers know the system has to change. Every four years, the annual General Conference on Weights and Measures (CGPM) brings together timekeeping authorities from around the world to discuss efforts to maintain UTC and its associated standards. In 2022 the 64 member states agreed to find a way to extend the allowed difference between UTC and Earth’s position from one second to a higher time limit by 2035.
Recent projections suggest that the need for a negative leap second is likely to hit before 2035. As the 2026 CGPM meeting approaches, the “leap hour” proposal could be the solution. Some experts say that a leap hour likely wouldn’t need to be added for hundreds of years, effectively abolishing the leap second.
Patrizia Tavella, director of the Time Department at the BIPM, says that there is sufficient urgency to put the leap hour into effect as soon as possible. The proposal under consideration would aim for it to go into effect next year.
“We estimated that if we wait till 2035, we have 30 percent risk of a negative leap second,” Tavella says. “So 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]?’ And they said, ‘No, even 10 percent risk is too much.’”
