The technical actions needed to ensure the continuity of UTC

Resolution 5 of the 28th CGPM mandates technical actions to preserve the long-term continuity of Coordinated Universal Time (UTC) by formally increasing the allowable maximum difference |UT1−UTC| in, or before, 2035 so that UTC remains continuous without future step adjustments for at least a century. The decision builds on earlier CGPM resolutions that define UTC’s relation to International Atomic Time (TAI) and its offset by integral seconds tied to Earth rotation (UT1). It argues that industrial users and national critical infrastructures - transportation, energy, telecommunications, space systems - require a continuous UTC, and highlights a 2025 expert estimate that the probability of a negative leap second rises rapidly, reaching about 30% by 2035. A negative leap second, never before applied, poses high risk of operational anomalies, substantial remediation costs comparable to the millennium bug, and growing vulnerability as IoT, distributed sensors, smart grids and virtual services proliferate.

The resolution specifies practical and coordination measures: UT1−UTC values are available through GNSS navigation messages and services from IERS and national metrology institutes; the ITU‑R is updating dissemination codes and WRC Resolution 655 allows a transition period to transmit larger UT1−UTC differences via radiocommunication until 2035 (but no later than 2040). It warns that inconsistent leap‑second mitigation by global providers creates timing mismatches and departures from the SI, and notes industry and standards bodies (ITU‑T, URSI, IEEE 1588) have urged accelerating adoption of a continuous UTC to avoid negative leap seconds and prevent migration to GNSS times that would weaken international metrological standardization. The resolution welcomes BIPM-ITU‑R collaboration and IERS engagement to implement the technical changes.