The practice of adding ‘leap seconds’ to official clocks to keep them in sync with Earth’s rotation will be put on hold from 2035, the world’s foremost metrology body has decided.
The decision was made by representatives from governments worldwide at the General Conference on Weights and Measures (CGPM) outside Paris on 18 November. It means that from 2035, or possibly earlier, astronomical time (known as UT1) will be allowed to diverge by more than one second from coordinated universal time (UTC), which is based on the steady tick 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.
Making the adjustments is “a big step forward” for researchers who focus on frequency and time, says Georgette Mcdonald, director of the Metrology Research Centre, Halifax, Canada. “I’m happy their efforts got us here .”
Leap second are unpredictable because they depend on the Earth’s natural rotation. They can disrupt systems that are based on precise timekeeping, Macdonald states, and can cause havoc in the digital age. Meta, Facebook’s parent company and Google are two of the tech companies that called for to stop.
The CGPM, which also oversees the international unit system (SI), has proposed that no leap second be added for at most a century. This would allow UT1 to drift out of sync by approximately 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 be allowed to diverge.
Time to change
Representatives from Canada, the United States and France were among those at the CGPM who called for the leap second to be scrapped before 2035. But Russia, which voted against the proposal, wants to push it back to 2040 or later to deal with technical issues within its satellite-navigation system, GLONASS.
The Russian system includes leap seconds, while others such as the Global Position System (GPS), and others effectively ignore them. According to Felicitas Arias, Felicitas Arias was the former director of the Time Department of the International Bureau of Weights and Measures in Sevres, France.
Astronomers who rely upon UT1 to align telescopes will also have to adjust, according to Elizabeth Donley, who heads the Time and Frequency division of the National Institute of Standards and Technology in Boulder, Colorado. She says that the current situation is not sustainable and will only get worse. Different organisations handle the leap second differently (Google, for example, smears out the extra second in the 24 hours around midnight UTC). She says this creates ambiguity between time sources that can be as high as half a second.
Although in the long term Earth’s rotation slows due to the pull of the Moon, a speed-up since 2020 has also made the issue more pressing, because for the first time, a leap second might need to be removed, rather than added. UTC has never had to slow down to wait for Earth. It did not have to skip ahead to catch up. Donley states that it is being called a Y2K problem because it’s something that we have never had to deal.
There is a chance that the International Telecommunications Union (ITU) could stymie plans to make the switch in 2035. The body effectively ceded decision making about the leap second to the CGPM in 2015, and Arias says its working group agreed with the CGPM’s proposal. She says that although the ITU retains control over disseminating UTC, it could argue that it is not the right time to make the change. “This is what makes us a little nervous .”
Although human timepieces have been calibrated to Earth’s rotation for millennia now, most people won’t feel any change from the loss of a leap second. Arias says that there is a one-hour difference between summertime and wintertime in most countries. It is more than one second, but it doesn’t affect you .”
Future metrologists may find more elegant ways to realign UTC/UT1 than the leap second. Macdonald says that when the difference becomes significant, “our capability to reconcile it” will be greater than what we have right now.
Or they may not bother, Arias says. She says that countries could shift their legal time zone one hour if the difference is large enough. We could also decouple our senses of time from the Sun, creating a single global time zone where different countries see the Sun overhead at different times. She says, “It could be the solution.” “Science doesn’t use local time, we use UTC .”
This article is reproduced with permission and was first published on November 18 2022.