It's Official: Timekeepers Have Gotten Rid of the Leap Second—and Introduced the Leap Hour Instead

Since 1972, experts have added 27 seconds to Coordinated Universal Time to keep it aligned with Earth’s rotation, but it has been an imperfect fix

Margherita Bassi | Daily Correspondent

October 15, 2026 4:43 p.m.

Every four years, February gets an extra day. You’ve heard of these “leap” years, especially if an acquaintance was born on the unusual date of February 29. What you might not have heard of, however, is the leap year’s sporadic sibling: the leap second.

This lesser-known timekeeping fix was center stage at this week’s General Conference on Weights and Measures in France, where timekeepers decided today to return it to obscurity by getting rid of it altogether.

Traditionally, one Earth orbit around the sun determines the length of a year, and one Earth rotation on its axis determines the length of a day. Because it takes Earth around 365.25 days to go around the sun, timekeepers add that extra leap day once every four years to make sure our seasonal calendar stays aligned with the orbit.

Earth’s daily rotation, however, isn’t as regular, because the speed of the planet’s spin can change. In general, Earth’s rotational speed has been slowing down throughout history. But starting around 2020, it has increased. A number of factors can play a role in Earth’s spin, including the atmosphere, ocean currents, wind, volcanoes and melting polar ice.

This makes Earth’s rotation a “lousy” clock, Jeff Sherman, a physicist at the United States’ National Institute of Standards and Technology (NIST), tells CNN’s Katie Hunt.

Even as Earth’s spin wavers, our atomic clocks—crucial to keeping ultra-precise time in this age of computers, phones, GPS and satellites—tick at a stable rate. That’s because in 1967, the General Conference on Weights and Measures chose a reliable way to define a second with the help of the isotope cesium-133. When a cesium-133 atom undergoes a change between specific energy states, it emits radiation that oscillates in an exceptionally regular manner, Vahe Peroomian, a physicist at the University of Southern California, writes for the Conversation. As such, scientists reclassified a second as equivalent to 9,192,631,770 cesium-133 atom oscillations.

Today, Coordinated Universal Time (UTC) follows this principal. But astronomical time, or Universal Time 1 (UT1), follows Earth’s fickle rotation, sometimes leading to discrepancies between the two time scales. To keep these systems as aligned as possible, timekeepers have added 27 leap seconds to UTC since 1972. “We effectively stop our clocks for that second to give Earth the opportunity to catch up,” as Konstantin Bikos and Anne Buckle write for Time and Date.

The goal has been for UTC and UT1 to differ by less than 0.9 seconds, and a leap second has been added when the misalignment approaches that limit. Today, however, experts at the General Conference on Weights and Measures voted to increase this margin to one hour. This negates the need for a leap second and creates the much rarer leap hour, which will be added when UTC deviates from UT1 by 3,600 seconds. Such a change likely won’t be needed for hundreds or thousands of years.

“It’s kind of a philosophical discussion, because if a leap hour happens, it won’t be for a very, very long time,” Christian Bizouard, director of the IERS Earth Orientation Center in France, tells the Guardian’s Ian Sample. “What we are doing is pushing back the decision.”

The move caps off 30 years of debate, as the leap second has been highly controversial in the world of timekeeping. Leap seconds aren’t nearly as smooth of a fix as leap years are, and they can cause computer and website issues. Reddit famously crashed when a leap second was added on June 30, 2012. It’s “always been a problem,” Judah Levine, a former researcher at the NIST’s Time and Frequency Division, tells Scientific American’s Emma Gometz.

Instead of adding the entire second all at once, companies such as Google and Meta add the extra time in smaller segments in a tactic called “smearing,” according to the Conversation. It’s not a simple task, however, and companies that employ smearing have long lobbied to get rid of the leap second.

What’s more, with Earth’s recent spin acceleration, timekeepers around the world were facing the possibility of the world’s first negative leap second, in which a second would be subtracted from UTC. This sparked fears, akin to those surrounding Y2K, that our modern technologies wouldn’t be able to handle such a change.

“It’s never happened before, and therefore the software for that has never been tested,” Darryl Veitch, a researcher who leads the Timing Lab at the University of Technology Sydney, tells the Australian Broadcasting Corporation’s Ellen Phiddian. But others argued that it wouldn’t be much of a problem.

The vote means that in the future, UTC will be allowed to diverge from astronomical time more than ever before.

“Astronomers, Earth scientists and other users of Earth-orientation data will need to account for the growing offset when relating their observations and measurements to civil time,” Peroomian writes in the Conversation. As for everyone else, hopefully whatever technology rules everyday life when a leap hour does eventually come around will be better equipped for the time fix.