International Timekeepers Propose Replacing the Leap Second with a Leap Hour by 2027

The International Bureau of Weights and Measures (BIPM) and other global timekeeping bodies are set to vote at the 2026 General Conference on Weights and Measures (CGPM) on a proposal that would replace the occasional “leap second” with a “leap hour.” The change aims to align Universal Coordinated Time (UTC) with Earth’s rotation while eliminating the need to add or subtract single seconds. If adopted, the new system could take effect as early as next year, rather than the previously discussed 2035 timeline.

Scientific American

Since 1972, leap seconds have been inserted whenever UTC drifted more than one second ahead of Earth’s rotation. The practice has caused disruptions for technology firms—Meta, Reddit, and Cloudflare experienced outages after a leap second insertion, and high-frequency trading systems have been affected. In 2016 Earth’s rotation began to speed up, halting the addition of positive leap seconds, but projections indicate a possible “negative leap second” before 2035, a scenario that could create unknown infrastructure risks.

Scientific American

Patrizia Tavella, director of the Time Department at the BIPM, emphasized the urgency: “We estimated that if we wait till 2035, we have 30 percent risk of a negative leap second.” She added, “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.’” Judah Levine, a former physicist with the U.S. National Institute of Standards and Technology, noted that the current leap-second system “has always been a problem.”

Scientific American

Proponents argue that a leap hour would likely not be needed for centuries, effectively ending the need for frequent adjustments. Critics warn that shifting to a larger correction interval could introduce new complexities for systems that rely on precise timestamps. The upcoming CGPM vote will determine whether the leap hour becomes the new standard for synchronizing global timekeeping with Earth’s rotation.

Scientific American