Earth's acceleration prompts shift from leap seconds to leap hours

International timekeeping experts agreed in March 2025 to replace leap seconds with a leap hour system. Leap seconds have kept atomic clocks in sync with Earth's rotation since the 1970s. They were needed almost every year at first, then every one or two years. But Earth has been rotating faster since around 2015, which has made the old system a problem. Experts now think there is a 30% chance that a negative leap second will be needed within ten years. This would mean removing a second from clocks, something computer systems cannot handle. A leap hour would only happen once every few centuries. The last leap second was added at the end of 2016.