The controversial leap second, a tiny adjustment to keep atomic time in sync with Earth's rotation, is officially dead. As of May 2027, the International Bureau of Weights and Measurements (BIPM) will cease adding these seconds, ending a debate that has simmered for decades and impacting critical technologies.
Since 1972, 32 leap seconds have been added to Coordinated Universal Time (UTC) to bridge the gap between hyper-accurate atomic clocks and the gradually slowing rotation of the Earth, influenced by factors like tidal friction from the moon. This adjustment was intended to keep UTC aligned with astronomical time (UT1), but it often caused headaches for digital systems.
While the concept might seem minor, the implementation of leap seconds has historically caused significant disruptions. Major tech companies like Google and Meta have employed workarounds, such as "smearing" the extra second over a 24-hour period, but other systems, like Qantas's booking system in 2012, have experienced critical failures.
Adding to the complexity, the Earth's rotational speed has recently increased, raising the possibility of negative leap seconds – a scenario with untested and potentially severe consequences. According to a 2024 Nature study, a negative leap second might have been necessary as early as 2029. The potential risks associated with negative leap seconds were a major factor in the decision to abolish them.
BIPM confirmed that the risk of a negative leap second played a crucial role in the decision, with tests showing significant failures when attempting to implement one. The retirement date of May 20, 2027, was chosen to coincide with the anniversary of the Metre Convention and Metrology Day. This change means systems can simply accumulate seconds without interruption, simplifying operations significantly.
While the leap second is gone, the discrepancy between atomic and astronomical time will continue to grow. BIPM has proposed a "leap hour" as a future solution, though this is expected to take millennia to accumulate. For now, users relying on UTC will see no immediate change, while those dependent on UT1 will need to manage the increasing difference.
