Since 1972, 27 leap seconds have kept Coordinated Universal Time, or UTC, aligned with UT1, the astronomical time scale based on Earth’s changing rotation. The General Conference on Weights and Measures is expected to approve a new limit supported by many national bodies. From 20 May 2027, UTC and UT1 could differ by up to one hour. The margin is intended to avoid further leap-second adjustments for centuries or longer.

Earth has rotated faster in recent years. That creates the possibility of a negative leap second, which would remove a second and has never been used. Experts estimate a 30 percent probability that global systems will require one by 2035. Many systems handle inserted seconds but are not designed for a second to disappear.

Setnam Shemar, principal scientist at the UK’s National Physical Laboratory, said incorrect leap-second handling can produce inconsistent timestamps across networks and contribute to failures in mobile phone networks or electricity grids. The change would preserve UTC’s relationship with UT1 for applications such as astronomy and celestial navigation. UT1 would remain under observation, and its difference from UTC would continue to be published through established international services.