World's Timekeepers Vote to Scrap Leap Seconds, Replace Them With a Leap Hour That May Not Come for Centuries

VERSAILLES, France — The world's official timekeepers voted Thursday to abolish the leap second, the occasional one-second adjustment that has kept atomic clocks in step with Earth's rotation for more than five decades, replacing it with a far rarer "leap hour."

The General Conference on Weights and Measures, the decision-making body of the International Bureau of Weights and Measures, approved a resolution allowing Coordinated Universal Time, or UTC, to drift from time based on Earth's rotation by up to one hour before any correction is required.

The change, adopted by a majority vote at the conference near Paris, will take effect on May 20, 2027, World Metrology Day.

"After 30 years of debates, we are ready," Noël Dimarcq, president of the bureau's Consultative Committee for Time and Frequency, said before the vote, according to Scientific American.

Why leap seconds exist

Modern timekeeping relies on atomic clocks, which measure seconds with extraordinary precision based on vibrations of cesium atoms. Earth's rotation, however, is irregular and gradually changing, so time based on the planet's spin, known as UT1, slowly drifts away from atomic time.

Since 1972, timekeepers have added leap seconds to UTC whenever the gap approached 0.9 seconds. In total, 27 leap seconds have been added, most recently on Dec. 31, 2016.

Under the new rules, the allowed difference between UTC and UT1 will grow from less than one second to 3,600 seconds, or one hour.

A leap hour far in the future

Because the drift accumulates slowly, scientists predict that a leap hour will not be needed for a very long time. Scientific American reported that a leap hour likely won't be needed for more than 1,000 years, while Time and Date said the change means no adjustments to clocks for possibly thousands of years.

"After 30 years of debate, the leap second is making way for the leap hour," said Setnam Shemar, a principal scientist at Britain's National Physical Laboratory.

Headaches for computers

Leap seconds have long caused problems for computer systems, which are not always designed to handle a minute with 61 seconds. Past leap seconds have triggered outages at websites and disruptions to software, and technology companies have developed workarounds such as gradually "smearing" the extra second over a day.

The issue became more urgent as Earth's rotation recently sped up slightly, raising the possibility that timekeepers would need to subtract a second for the first time, known as a negative leap second.

Patrizia Tavella, director of the time department at the International Bureau of Weights and Measures, has warned that because a negative leap second has never been tested, "the problems it could create are without precedent."

Estimates put the probability of a negative leap second within the next decade at about 30%, according to reports, adding pressure for the change.

Years of debate

The decision caps decades of discussion among scientists, governments and industry groups. In 2022, member states agreed to find a way to extend the allowed difference between UTC and Earth's rotation by 2035. Thursday's vote settles how that will be done.

Some astronomers and others have argued that civil time should stay closely tied to Earth's rotation, which has historically defined the day. Supporters of the change say the practical benefits of a continuous time scale outweigh the small and slowly growing gap.

What it means for everyday life

For most people, the change will go unnoticed. Clocks, phones and computers will keep running as usual, and the difference between atomic time and solar time will remain far too small to affect daily life for generations.

The biggest beneficiaries are likely to be technology companies, telecommunications networks, financial markets and navigation systems, which depend on precise and continuous timekeeping.

What's next

Timekeeping authorities will prepare to implement the new framework before May 2027. Decisions about how and when a future leap hour might be applied will be left to future generations.