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The leap second is dead. Long live the leap hour?
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A negative leap second could mean computer systems no longer run like clockwork. Credit: Florian Gaertner/Photothek/Getty
The leap second could be scrapped as soon as October. The periodic adjustment to the world’s clocks, which has been done every few years since 1972, was already set to end by 2035. But 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. This has never happened before, and comes with unprecedented risk, say researchers.
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 the timescales only once they differed by 3,600 seconds. 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.
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.
But the addition of a leap second — which last happened in 2016 — can cause havoc for the computing systems that rely on time synchronization to operate correctly. These include crucial infrastructure such as electrical grids, telecommunications, financial systems and the Internet.
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.
And the next adjustment could be worse. Leap seconds are needed because Earth’s rate of rotation varies. 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 time scales 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 have to skip, rather than gain, a second, so that official time can catch up with time according to Earth’s spin. This has never happened before, and comes with unprecedented risks. A negative leap second could disrupt “critical infrastructures”, and would be expensive to mitigate against, according to the International Bureau of Weights and Measures, which published the resolution. “There is an apprehension about what equipment failures might happen,” says Macdonald.
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.