Global timekeeping delegates voted on October 15 to end leap-second adjustments to Coordinated Universal Time (UTC) from May 20, 2027, allowing it to drift as much as an hour from time measured by Earth’s rotation.
The decision, adopted by the General Conference on Weights and Measures (CGPM) in Versailles, brings forward a change first agreed in 2022, when delegates set 2035 as the target for widening the allowable gap between UTC and Earth-rotation time, known as UT1. UTC is calculated from atomic clocks, while UT1 tracks Earth’s variable spin; leap seconds have been added to keep the two time scales close. Since the practice began in 1972, 27 leap seconds have been added, most recently at the end of 2016.
But Earth’s recent faster rotation raised concern that timekeepers might soon have to subtract a second for the first time—a scenario digital systems have never had to handle. Officials said abrupt time adjustments pose risks for infrastructure such as telecommunications, energy networks, transport and space systems. From 2027, the permitted difference between UTC and UT1 will rise to 3,600 seconds, or one hour.
The change does not mean clocks will jump forward by an hour in 2027; the gap is expected to take centuries to approach that size, and users who need Earth-rotation time will still be able to access UT1 data.