International Metrology Body Votes to Eliminate Leap Seconds Starting in 2027

In an overhaul of global timekeeping, delegates at the 28th General Conference on Weights and Measures in Versailles, France, voted to abolish the leap second starting May 20, 2027. The historic resolution, passed at the Palais des Congrès, decouples Coordinated Universal Time from minor fluctuations in Earth's axial rotation, allowing atomic clocks to run continuously without intermittent, manually added seconds. The implementation date was selected symbolically to mark World Metrology Day, commemorating the founding of the Metre Convention on May 20, 1875.

Since 1972, international metrologists have inserted leap seconds into Coordinated Universal Time to keep it synchronized with astronomical time, known as UT1. While modern atomic clocks maintain time with unwavering precision, Earth's rotational speed fluctuates due to movements in its liquid core, atmospheric shifts, and tidal forces from the moon and sun. To ensure the gap between atomic time and Earth's rotation never exceeded 0.9 seconds, officials have added 27 leap seconds over the past five decades, most recently at the end of 2016.

Although adding a single second might seem imperceptible in everyday life, it has posed an escalating hazard to modern digital infrastructure. Software systems, telecommunications, energy grids, financial exchanges, and satellite navigation networks rely on unbroken, uniform time synchronization. A single irregular second has historically triggered major software crashes. In 2012, an added leap second disrupted thousands of Linux-based data centers worldwide and crippled the check-in and booking systems of Australian airline Qantas, delaying or canceling approximately 400 flights.

The conference had originally contemplated delaying the phaseout until 2035, but changing geophysical conditions prompted urgent action. In recent years, Earth has experienced a temporary acceleration in its rotational speed. Metrologists warned that this acceleration could necessitate the first-ever negative leap second—a scenario where a second would be subtracted, forcing clocks to skip directly from 23:59:58 to 00:00:00. Because such a subtraction had never been tested in live environments, scientists feared catastrophic failures across global software systems that lack standardized protocols for omitted time.

Under the approved reform, the allowable divergence between atomic time and astronomical time will expand significantly from 0.9 seconds to as much as 3,600 seconds, or one full hour. Experts estimate that this buffer will allow Coordinated Universal Time to operate uninterrupted for centuries or even thousands of years without requiring manual corrections. If an adjustment ever becomes necessary millennia from now, metrologists propose using a scheduled leap hour, which can be managed seamlessly in a manner akin to standard daylight saving time transitions.

"With the Versailles resolutions, we bring the reliability of the SI to where machines exchange data and where AI prepares decisions," said Professor Cornelia Denz, president of Germany's national metrology institute, the Physikalisch-Technische Bundesanstalt. Denz was re-elected in Versailles to the International Committee for Weights and Measures, representing 69 member states and 34 associate nations. Beyond the leap second, the conference adopted measures to standardize machine-readable digital units and digital calibration certificates, while establishing a new observer status to allow non-member countries and international organizations like the World Meteorological Organization greater access to international metrology.

For ordinary citizens using smartphones, computers, and conventional clocks, the shift will occur entirely behind the scenes with no noticeable disruption. Most people will never perceive the fractional divergence from the sun's position, as standard time zones and daylight saving adjustments already deviate from exact solar noon. However, the reform sets the stage for further scientific milestones: by 2030, metrology institutions aim to redefine the standard second using advanced optical atomic clocks rather than traditional cesium atoms. Furthermore, international experts are working on a dedicated Lunar Coordinate Time to manage gravitational time variations during upcoming lunar exploration missions.