Global Timekeepers Weigh Replacing Leap Second With Leap Hour

Earth’s increasingly quirky spin may soon trigger a miniscule but important makeover for the world’s clocks. International timekeepers are weighing a plan to scrap the occasional “leap second” and instead allow time to drift until a full leap hour is needed—likely centuries from now, reports Scientific American.

General Conference on Weights and Measures Resolution and October Vote

Member states of the General Conference on Weights and Measures (CGPM) will vote on the resolution at their 28th meeting, outside Paris on 13–15 October. The General Conference on Weights and Measures meets every four years, per Gizmodo. If it passes, the plan could go into effect as early as 2027.

Leap seconds have been in use since 1972 to keep Coordinated Universal Time in sync with Earth’s rotation. An extra second has been slipped into Earth’s official Coordinated Universal Time (utc) 27 times since the 1970s. Each one enables utc, which is maintained using atomic clocks, to be aligned with time according to the planet’s naturally varying rotation, which served as the world’s timekeeper for millennia. While the periodic adjustment to the world’s clocks was already set to end by 2035, authorities are considering an immediate end to avoid the real possibility that the next leap second will be negative — meaning that clocks must skip, rather than gain, a second.

The Threat of a Negative Leap Second and Planetary Spin

This has never happened before, and comes with unprecedented risk, say researchers. On the scale of millions of years, the planet’s spin is slowing, owing to friction from the pull of the Moon. But on shorter timescales, it fluctuates. Since around 2015, it has been speeding up, a change that has been attributed to shifting currents in the planet’s liquid core.

As a result, say scientists who study the planet’s rotation, there is a 30% probability that a negative leap second will be needed by 2035. In this scenario, utc would for the first time have to skip, rather than gain, a second, so that official time can catch up with time according to Earth’s spin. A negative leap second could disrupt critical infrastructures, and would be expensive to mitigate, according to the International Bureau of Weights and Measures, which published the resolution. There is an apprehension about what equipment failures might happen, says Georgette Macdonald, director-general of the Metrology Research Centre in Halifax, Canada.

Timekeepers propose leap hour to replace leap second by 2035

Technical Discontinuities and Corporate Workarounds

The addition of a leap second — which last happened in 2016 — has long vexed computers, contributing to outages at companies including Meta and Reddit, and complicating high-speed trading, telecommunications, electrical grids, financial systems, the Internet, and aviation systems.

Part of the problem with the current system is that there are several ways in which organizations cope with leap seconds. Tech giant Google, for example, chooses to “smear” out the extra second gradually throughout the day, whereas other companies opt to add a whole second to a specific minute. Some financial exchanges adjust their trading times to avoid having markets operating during the 61-second minute in which the extra second is added. We end up with this discontinuity in a system that's intended to be continuous, says Georgette Macdonald, director-general of the Metrology Research Centre in Halifax, Canada.

Implementing a Leap Hour Alternative

To avoid future chaos, the world’s timekeepers are for the first time proposing that the leap second be replaced by a leap hour, which would realign timescales only once they differed by 3,600 seconds. Aligning the timekeeping systems only after they have drifted apart by an hour has several advantages, says Macdonald. It wouldn’t occur for hundreds, if not thousands, of years, she says.

And having a solar day that differs from a day dictated by atomic time might not be as problematic as people imagine; adding or removing an hour already happens in countries that use daylight-saving time, so a leap hour should be much more manageable than a leap second. From an implementation perspective, the hour step change is something people are more familiar with, says Macdonald.

A leap hour might still present some technical difficulties, but financial systems, for example, are likely to operate quite differently thousands of years from now, says Macdonald. We might not have the same considerations then as we do today, she adds.