Time for the leap second is up: the world votes to stop pausing clocks

The practice of adding “leap seconds” to official clocks to keep them synchronized with the Earth’s rotation will be suspended from 2035, the world’s leading metrology body has decided.

The decision was made by representatives of governments from around the world at the General Conference on Weights and Measures (CGPM) held in Paris on November 18. It means that from 2035, or perhaps earlier, astronomical time (known as UT1) could deviate by more than a second from Coordinated Universal Time (UTC), which relies on the constant ticking of atomic clocks. Since 1972, whenever the two time systems have drifted apart by more than 0.9 seconds, a leap second has been added.

Stopping the adjustments is “a leap forward” for time and frequency researchers, says Georgette Macdonald, general manager of the Metrology Research Center in Halifax, Canada. “I am delighted that their efforts have brought us to this moment.”

Leap seconds are not predictable, because they depend on the natural rotation of the Earth. They disrupt systems based on precise timekeeping, says Macdonald, and can wreak havoc in the digital age. Facebook’s parent company Meta and Google are among the tech companies that have called for leap seconds to be scrapped.

The CGPM, which also oversees the International System of Units (SI), has proposed not adding any leap seconds for at least a century, allowing UT1 and UTC to slip out of sync by about 1 minute. But it plans to consult with other international organizations and decide by 2026 on what upper limit, if any, to put on how much they can diverge.

Time to change

Representatives from Canada, the United States and France were among those at the CGPM calling for the leap second to be abolished before 2035. But Russia, which voted against the proposal, wants to postpone it to 2040 or beyond to address technical issues within of its satellite navigation system, GLONASS.

The Russian system incorporates leap seconds, while the Global Position System (GPS) and others already effectively ignore them. The decision means Russia may need to install new satellites and ground stations, says Felicitas Arias, former director of the weather department at the International Bureau of Weights and Measures (BIPM) in Sèvres, France.

Astronomers who rely on UT1 to align their telescopes will also have to adjust, says Elizabeth Donley, who heads the Time and Frequency Division at the National Institute of Standards and Technology, in Boulder, Colorado. But the current situation is unsustainable and would continue to get worse, she adds. Different organizations handle the leap second differently (Google, for example, spreads the leap second into the 24 hours around midnight UTC). This creates an ambiguity between the time sources of up to half a second, she says, “which is huge.”

While Earth’s rotation will slow down in the long run due to the pull of the Moon, an acceleration since 2020 has also made the problem more pressing, because for the first time, a leap second may need to be removed, rather than added. UTC only had to slow down a beat to wait for Earth, not jump ahead to catch up with it. “It’s kind of like being described as a year 2000 problem, because it’s just something we’ve never dealt with,” says Donley.

There is a possibility that the International Telecommunication Union (ITU) could thwart plans to make the switch in 2035. The body effectively handed over decision-making on the leap second to the CGPM in 2015, and Arias says his group of work agreed with the CGPM proposal. But the ITU maintains control of UTC deployment and may argue that now is not the right time to make the change, he says. “That’s the thing that makes us a little nervous.”

Subtle difference

Although human clocks have been calibrated with the Earth’s rotation for millennia, most people will feel little effect from the loss of the leap second. “In most countries, there’s a one-hour shift between summer and winter time,” Arias says. “It’s much more than a second, but it doesn’t concern you.”

Future metrologists may find more elegant ways than the leap second to realign UTC and UT1. As the difference becomes significant, “our ability to reconcile it will be better than our ability is right now,” says Macdonald.

Or they might not care, Arias adds. When the difference becomes large enough, countries could permanently shift their DST by one hour, she says. Or we could even completely decouple our sense of time from the Sun, to create a single world time zone where different countries see the Sun overhead at different times of the day or night. “It could be a solution,” she says. “Science already doesn’t use local time, we speak in UTC.”

This article is reproduced with permission and was first published November 18, 2022.

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