Leap Hour Replacing Leap Seconds Due to Earth's Rotation Changes

Global timekeepers are set to abandon the long-standing practice of the leap second, opting instead for a more infrequent "leap hour." The decision, driven by concerns over the accelerating rotation of the Earth and the increasing complexity of managing atomic clocks, was largely agreed upon by experts at the World Radiocommunication Conference (WRC-2026) in Geneva on October 15, 2026. The shift aims to simplify time coordination between increasingly precise atomic time scales and the slightly variable time based on Earth's rotation.

For decades, scientists have inserted an extra second, known as a leap second, into atomic time (TAI) to keep it within 0.9 seconds of coordinated universal time (UTC), which is tied to Earth's rotation. This adjustment has occurred 27 times since its inception in 1972. However, recent measurements indicate that Earth's spin is not slowing down as much as previously thought, and in some periods, it has even accelerated, making the leap second system more erratic and difficult to manage, particularly for the digital infrastructure reliant on precise time synchronization.

The Challenge of Earth's Shifting Rotation

The Earth's rotation is not constant; it fluctuates due to various factors, including tidal friction from the Moon, seismic activity, and atmospheric changes. For a long time, the prevailing theory was that Earth's rotation was gradually slowing down, necessitating periodic additions of leap seconds to keep astronomical time (UT1) aligned with atomic time. However, since 2020, observations have shown a contrary trend, with the Earth spinning faster, bringing UTC closer to UT1 without the need for adding seconds. This acceleration means that if leap seconds were still being added at the old pace, the difference could eventually require a negative leap second, a concept that poses significant challenges for digital systems.

Dr. Peter Whibberley, a senior researcher at the UK's National Physical Laboratory, highlighted the complexity. "The Earth's rotation is speeding up, and we might need a negative leap second in the future," he stated earlier this year. "A negative leap second is complicated to implement. We want to avoid that." The current system of adding seconds has caused disruptions in the past, with major internet companies like Google and Amazon reporting issues with their systems during leap second insertions, sometimes leading to outages or data inconsistencies.

The move away from leap seconds is expected to usher in a new era of timekeeping. The WRC-2023 had already paved the way for this change, and the consensus at WRC-2026 solidifies it. The proposed "leap hour" would be a much larger adjustment, likely occurring only once every few centuries, making it far less disruptive to modern digital infrastructure. This change will ensure that the difference between atomic time and astronomical time does not exceed a significant margin, perhaps around 30 to 60 minutes, before a leap hour is implemented.

The International Telecommunication Union (ITU), the United Nations agency responsible for information and communication technologies, is overseeing the transition. The new system is anticipated to be fully implemented by 2035. This decision by international scientists aims to balance the needs of precise scientific measurements with the practical requirements of a globally connected digital world, ensuring that timekeeping remains stable and reliable for generations to come.