IERS says no leap second in December 2026

The International Earth Rotation and Reference Systems Service (IERS) said in Bulletin C 72 that no leap second will be introduced at the end of December 2026. The notice, issued in Paris on 6 July 2026, confirms that UTC will continue to run without an extra one-second adjustment at the next scheduled decision point.

Leap seconds are the awkward bridge between two different ways of keeping time. International Atomic Time (TAI) is built from atomic clocks and advances uniformly; Coordinated Universal Time (UTC) is the civil time scale used worldwide, but it is periodically nudged so it does not drift too far from Earth’s actual rotation (tracked as UT1). When Earth’s rotation slows or speeds up relative to the atomic standard, the gap between UTC and UT1 changes, and the IERS can recommend inserting a leap second at the end of June or December.

In its bulletin, the IERS restated the current offset between UTC and TAI: from 1 January 2017 until further notice, UTC–TAI equals minus 37 seconds. That figure matters because it is the fixed correction used by navigation systems, scientific instruments, financial systems and telecommunications networks that need to translate between time scales. A leap second would change the offset by one second, forcing every system that assumes a smooth progression of timestamps to accommodate a deliberate discontinuity.

The bulletin also underlines how the decision process works: Bulletin C is issued every six months, either to announce a time step or to confirm that none will occur at the next possible date. The trigger is the “evolution of UT1–TAI” — a technical way of saying that the IERS is watching whether Earth’s rotation is drifting far enough from atomic time to justify intervention.

For operators of time distribution — national metrology institutes, telecom providers, satellite operators and data centres — “no leap second” is itself operational guidance. It reduces the risk of software bugs, duplicated timestamps, and the kind of cascading failures that can follow from a one-second anomaly in systems built for continuous time.

The IERS bulletin was signed by Christian Bizouard, Director of the Earth Orientation Center at the Observatoire de Paris. It ends with a simple statement: no leap second will be introduced at the end of December 2026.