The 28th meeting of the General Conference on Weights and Measures (CGPM) has approved Resolution 5, which will end the practice of inserting leap seconds into Coordinated Universal Time (UTC) starting 20 May 2027. The resolution, adopted at the conference in 2026, sets a maximum allowable difference of 3,600 seconds (one hour) between UTC and the Earth's rotational time (UT1) to preserve long-term continuity of the time scale.

The decision follows years of consultation with industry groups, international standards bodies, and technical experts. The International Bureau of Weights and Measures (BIPM), which oversees UTC, cited growing demands from sectors such as telecommunications, energy distribution, transportation, and space operations for a stable, uninterrupted time reference. These users require precise synchronization without the risk of abrupt adjustments that could disrupt systems.

A key concern driving the change is the increasing likelihood of a negative leap second—where a second would be removed from UTC to align with Earth's rotation. Experts estimate a 30% probability of such an event by 2035. Unlike positive leap seconds, which have been used periodically since 1972, a negative insertion has never been tested and could pose significant risks to infrastructure reliant on time synchronization, including IoT networks and smart grids.

The BIPM will continue to collaborate with organizations such as the International Telecommunication Union (ITU), the International Earth Rotation and Reference Systems Service (IERS), and the International Astronomical Union (IAU) to inform stakeholders and support implementation. The organization will also monitor UT1-UTC values and provide regular updates on their evolution.

The resolution does not eliminate the need for timekeeping precision but shifts the responsibility for maintaining alignment to external references. Users seeking to track Earth's rotation can still access UT1 data through GNSS navigation messages and IERS services. The transition will allow timekeeping systems to operate without leap seconds while preserving the integrity of UTC as a global standard.

The official text of Resolution 5 is available in French, the working language of the CGPM.