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World’s timekeepers vote to replace the leap second with a leap hour October 16, 2026 · 4:37 · Science and the planet · With Maya and Nora
Today, we’re asking why the world’s timekeepers have voted to change how they adjust the clocks we use around the world.
Transcript Maya Welcome to Slow English FM. This is Science and the Planet, where we explore questions about science and the world around us in clear English. I’m Maya, and Nora is our columnist for science and the environment. Today, we’re asking why the world’s timekeepers have voted to change how they adjust the clocks we use around the world. Maya Nora, what did the delegates decide? Nora The Guardian reports that delegates from 69 countries voted at a conference in Versailles, France, on 15 October 2026. They agreed to replace leap seconds with a system that allows a much wider gap between two ways of measuring time. The change is due to begin the following year. Maya Let’s unpack that. What is a leap second, and what does the new system change? Nora The clocks used for everyday timekeeping are based on atomic clocks, which keep time very steadily. But the Earth’s rotation is not perfectly steady. Astronomers track the position of the Sun to measure that rotation. A leap second has been added to clock time when needed to keep these measures close. The new approach lets the gap grow much larger before a correction is needed. The Guardian says the difference could reach up to an hour. It also reports that this limit is unlikely to be reached for thousands of years. So the vote does not mean that clocks will suddenly add an hour next year. Maya So this is about leaving more room before a correction, not changing the clocks by an hour right away? Nora Exactly. The change is to the limit for the difference between the two time measures. It is not an instruction to make a one-hour change as soon as the new system starts. Maya Why make that change? Nora The Guardian reports that the main concern is the risk of leap seconds causing problems for computer systems and other technical equipment. These systems often depend on time stamps: records that show when an event happened. A sudden change to the clock can be difficult for some systems to handle. The Guardian quotes Setnam Shemar, a scientist at the UK’s National Physical Laboratory. He says some computer systems could not handle the extra second added in 2012. Some servers froze or went offline, he said. That is his account of what happened, and it helps explain why timekeepers are concerned about these adjustments. Maya The Guardian also says the vote was brought forward because a negative leap second could be needed. What does “negative” mean here? Nora It would mean removing a second rather than adding one. The Guardian says the possibility of a negative leap second helped speed up the timetable. It does not give a specific probability. Maya Was this a completely new plan, or had timekeepers already agreed to move away from leap seconds? Nora There was already an earlier decision. In a 2022 resolution, the International Bureau of Weights and Measures set a goal to increase the permitted difference between the time measures in or before 2035. The Guardian reports that the 2026 vote brings the change forward to the following year. Maya What is the wider point for people who do not work with clocks or computer networks? Nora Many technical systems use precise time records, and the decision is intended to reduce the risks linked to sudden clock adjustments. In my view, the key idea is that keeping time is not only about setting clocks. It also means keeping the systems that rely on those clocks working together. Maya Nora, thank you. And thanks to you for listening to Science and the Planet on Slow English FM. We’ll leave it there.
