The General Conference on Weights and Measures (CGPM) has scheduled continuous Coordinated Universal Time (UTC) to begin on May 20, 2027, and set a one-hour maximum for the difference between UTC and Earth-rotation time UT1. The decision, in Resolution 5, is intended to avoid future leap-second adjustments while keeping a shared time standard for systems that depend on precise synchronization.

UTC and Earth-rotation time will diverge

UTC is produced by the Bureau International des Poids et Mesures (BIPM) at the rate of International Atomic Time (TAI), with an offset measured in whole seconds. UT1, by contrast, tracks Earth’s angular rotation. Leap seconds have been used to keep UTC close to UT1; under Resolution 5, the absolute difference, written |UT1−UTC|, can reach 3,600 seconds, or one hour.

The change is scheduled for May 20, 2027, not before. The resolution sets the maximum permitted difference as UTC continues without irregular leap-second adjustments.

Why continuous UTC matters to infrastructure

The CGPM cites transportation, energy distribution, telecommunications and space projects as users that need continuous UTC for synchronization. A negative leap second would remove a second rather than add one; the resolution says none has previously been applied and that critical infrastructure is largely unprepared for it.

A workshop organized by the Consultative Committee for Time and Frequency (CCTF) and the International Earth Rotation and Reference Systems Service (IERS) in March 2025 estimated a 30% probability of a negative leap second by 2035. The resolution also says preparation costs were estimated to be comparable to preparations for the millennium bug.

The resolution points to another operational problem: providers use different leap-second “smearing” methods, which spread an adjustment over time. Those methods can produce inconsistent timing across systems and in disseminated time and frequency. A continuous UTC is intended to avoid those conflicting adjustments while preserving one internationally standardized time scale.

Where to get UT1 information

Continuous UTC does not remove the need for Earth-rotation data. Resolution 5 identifies GNSS navigation messages and services from IERS, national metrology institutes and other providers as ways to obtain UT1–UTC information.