Leap seconds have helped keep clocks synchronized with Earth’s rotation, but their time may be over – here’s why

Since 1972 timekeepers have added 27 leap seconds to UTC to keep atomic-clock time aligned with Earth's actual rotation, which runs about four milliseconds short of the 86,400-second atomic day. The last leap second was added in 2016, and in October 2026 an international timekeeping body may change the strategy going forward.

The piece traces the history from ancient Egyptian hour divisions through the 1967 adoption of the cesium-133 second, and notes that leap seconds have caused real outages, including a 2012 Reddit crash and disruptions to Linux and Java systems, prompting companies like Google and Meta to 'smear' the extra second instead of inserting it all at once.

The full dispatch is available from the source below.