Delegates from 69 countries voted on Thursday at a conference in France of the International Bureau of Weights and Measures to adopt a leap hour as a replacement for the leap second. The decision aims to reduce risks to computer systems and infrastructure that depend on precise timing, while preserving the link between global time and Earth’s rotation.

Dr Setnam Shemar, principal scientist in the time and frequency department at the UK’s National Physical Laboratory, said the change follows 30 years of debate. "This will reduce the risks to the systems and national infrastructure that depend on accurate time while still preserving the fundamental link between our global time system and the rotation of the Earth," he said.

The vote permits coordinated universal time to drift up to an hour from astronomical time before a leap hour is required, an adjustment unlikely to occur for thousands of years. Unlike the familiar leap year, which adds a day every four years to align the calendar with Earth’s orbit, leap seconds were introduced more recently to keep atomic time in sync with Earth’s slowing rotation.

Since 1972, 27 leap seconds have been added to keep UTC aligned with astronomical time. However, recent observations show Earth’s rotation has accelerated, raising the possibility of a negative leap second — removing a second from UTC. A 30% probability of such an event within the next decade prompted the accelerated vote.

Leap seconds have previously caused technical disruptions. After the 2012 leap second, systems including Reddit and LinkedIn experienced outages, and Qantas flights faced delays. Negative leap seconds pose greater risks, as no software has been tested against them. Testing global systems for such an event would require substantial investment, a factor that influenced the decision.

The UK has historically opposed leap seconds due to their connection with Greenwich Mean Time, while Russia raised concerns about impacts on its Glonass satellite system. The International Earth Rotation and Reference Systems Service in Paris will continue monitoring Earth’s rotation and issuing notices when adjustments are needed. The shift to a leap hour defers the final decision to future experts, as the change is not expected for many millennia.