The international authority responsible for timekeeping has voted to eliminate the leap second, a practice used since 1972 to adjust atomic clocks in line with irregularities in Earth’s rotation. The change, decided during a meeting held in France, will take effect in 2027.

The leap second was originally introduced to synchronize Coordinated Universal Time (UTC) with the planet’s variable rotational speed, which does not follow a perfectly predictable pattern. Unlike the leap year, which adds an extra day every four years to compensate for Earth’s orbit around the sun, leap seconds have been inserted sporadically to adjust for unpredictable fluctuations in Earth’s spin.

The decision to abolish the leap second arose partly due to emerging concerns about the potential need for a “negative leap second.” This would occur if Earth’s rotation sped up enough to require clocks to be set back by a second, a scenario never before encountered and one that could risk critical infrastructure stability. Noël Dimarcq, president of the International Bureau of Weights and Measures (BIPM) committee for time and frequency, said recent changes in Earth’s rotation made this possibility increasingly relevant.

Instead of periodic one-second adjustments, the BIPM plans to extend the interval between corrections, deferring synchronization until a full hour’s correction is necessary. Patrizia Tavella, director of BIPM’s time department, described this new approach as simpler to manage. Experts estimate that such an extended correction might not occur for hundreds or even thousands of years.

Historically, Earth’s rotation served as the primary global time standard. Since the mid-20th century, the advent of atomic clocks, which measure consistent atomic vibrations, allowed for far more precise timekeeping. This led to the adoption of UTC in 1963, providing a universal standard for telecommunications, navigation, and computing.

However, as atomic clocks became more accurate, the subtle variations in Earth’s rotational speed became more apparent, necessitating leap seconds to maintain alignment. These adjustments, while intended to preserve synchronization, have intermittently posed problems for digital systems. The insertion of a leap second can cause technical difficulties, even leading some tech companies to employ alternative methods such as “leap smearing,” which slows down clocks gradually rather than inserting a leap second abruptly.

Another complication stems from the fact that Earth’s rotation does not always slow down; gravitational interactions with celestial bodies and internal Earth's dynamics can accelerate it. While many systems are designed to handle the insertion of a leap second by pausing or repeating a second, far fewer have protocols for subtracting a second, raising concerns about negative leap seconds.

The vote to discontinue the leap second was not unanimous. Delegates from Russia, whose satellite systems are equipped to handle leap seconds, opposed the measure, while Belarus chose to abstain. The current discrepancy between atomic time and Earth’s rotation is near zero, suggesting that the next leap second could have been negative, highlighting the challenges faced by the existing system.

With the leap second removed, global timekeeping will rely solely on atomic clocks until a larger adjustment becomes necessary, simplifying synchronization for modern technological infrastructures while postponing the challenge of realigning time with Earth’s rotation for potentially many centuries.