The future of timekeeping and the use of leap seconds

3 minute read

Today (15 October 2026) at the General Conference on Weights and Measures (CGPM) a vote passed to move to a continuous global time scale, avoiding the need for leap seconds in future.

Leap seconds have been used since 1972 to keep Coordinated Universal Time (UTC) in step with the astronomical time scale (UT1), which is based on the Earth’s rotation. The Earth’s rotation is not perfectly regular and fluctuates due to natural influences. It has historically slowed down gradually, so the occasional addition of leap seconds has been necessary to prevent UTC drifting away from astronomical time. 

Unusually, the Earth's rotation has accelerated in recent years, creating the additional possibility that a negative leap second may be required for the first time. Current estimates suggest around a 30 % probability of this occurring within the next decade.

The decision taken today by the CGPM reduces the risk to systems that depend on precise and continuous time synchronisation, including national infrastructure that we rely on daily. Leap seconds can be difficult to implement reliably, and if not applied correctly they can cause differences in time across networks, potentially leading to the failure of systems such as mobile phone networks or electricity grids. These risks could increase if a negative leap second had to be implemented for the first time. Today’s decision therefore removes uncertainty for those responsible for implementing and maintaining time-dependent infrastructure. 

Previously, leap seconds were added to UTC to keep it within 0.9 seconds of UT1. From 20 May 2027 a difference of up to 1 hour will be allowed before intervention is needed, ensuring the continuity of UTC for several centuries or more. 

The relationship between UTC and UT1 is important for some applications, such as astronomy and celestial navigation. Today’s decision does not sever that link: UT1 will continue to be monitored and its difference from UTC will continue to be disseminated through established international services.