World's timekeepers to vote on scrapping the leap second

Delegates from the International Bureau of Weights and Measures (BIPM), the body that standardises the world's measurements, will meet near the palace of Versailles from 12 to 15 October to decide whether to retire the leap second permanently. Under the leading proposal, coordinated universal time (UTC) would be allowed to drift away from Earth's rotation until it is an hour out of sync, at which point a single leap hour would bring it back into line. Because the planet currently loses less than a second of rotational time per year, that correction may not be needed for more than a thousand years.

Leap seconds exist because a second was traditionally defined as an 86,400th of a day, yet Earth's rotation is not uniform and daily lengths can vary by a millisecond. From the late 1960s, the BIPM adopted a more precise definition based on caesium atoms, which now underpins UTC and is tracked by atomic clocks — with newer nuclear clocks expected to eventually surpass them. Since leap seconds were introduced in 1972, 27 have been added, the last at the end of 2016, in order to keep the two timescales together.

The instability has several causes. University of Tasmania geodesist Lucia McCallum told the ABC that the atmosphere, winds, large ocean currents, melting glaciers, volcanoes and the redistribution of mass inside the Earth can all affect the planet's spin. Over the long term the rotation is slowing, largely because of the Moon's pull — a slowdown astronomers identified by comparing written records of solar eclipses from ancient China and the Middle East more than 3,000 years ago with astronomical models. More recently, since about 2020, Earth has been spinning faster.

That speed-up created a problem: the next adjustment might have had to be a negative leap second, removing rather than adding a second. Michael Wouters, who leads time standards at Australia's National Measurement Institute, said a negative leap second would be less likely to cause trouble because it would make time jump forwards rather than backwards. Darryl Veitch, a computer networking researcher at the University of Technology Sydney, was less confident, noting it has never happened and the software for it has never been tested. According to the ABC, the risk prompted the BIPM's governing body to accelerate a timetable it had already agreed in 2022, when it decided to abandon the leap second by 2035.

Leap seconds are irregular and scheduled only about six months in advance, unlike time zones or daylight saving. Veitch said they violate assumptions built into software, including that time only moves forwards and that a minute contains 60 seconds, while Wouters said programmers forgot about the issue and wrote code that crashed when clocks went backwards. Platforms including Cloudflare and Reddit have previously been hit by such failures, and increasingly interconnected systems mean a timekeeping error in one place can snowball.

Wouters said there are no clear economic or practical arguments for keeping leap seconds, beyond keeping software engineers occupied, and described opposition as largely philosophical — a discomfort with disconnecting how time is counted from how it is experienced. He also noted a personal upside: leap seconds were often added on 31 December, meaning some National Measurement Institute staff worked on New Year's Day to check for measurement problems. Veitch predicted that in a century software will be fully automated, while McCallum said it makes no difference whether UTC runs seconds behind the Earth's days.