The Leap Second Has Been Cancelled. It’s About Time

The leap second, once introduced to reconcile atomic time with Earth's changing rotation, has been cancelled due to complications it poses for digital systems.

By the mid-20th century, the advent of computers, international communications systems, and space exploration demanded greater timekeeping precision. This led to the creation of atomic clocks, which measure time by tracking the tiny, regular changes atoms undergo. These ultra-precise instruments ushered in a global standard of timekeeping known as Coordinated Universal Time, or UTC, which was adopted in 1963.

However, subtle shifts in Earth’s rotation caused by gravitational interactions with celestial bodies or the movements of air, water, and rock became more noticeable once clocks grew significantly more precise. To account for these discrepancies, authorities adopted the leap second. In recent decades, though, adding this single second created two main problems for digital systems.

Adjusting time by a single second can prove to be a hassle and even cause time-linked systems to temporarily shut down. To mitigate this issue, tech giants like Google adopted a leap smear, temporarily slowing down their clocks instead of adding a sudden second. Furthermore, Earth’s rotation does not always lag behind, meaning forces could eventually require a negative leap second.

While most digital systems relying on UTC are prepared to jump ahead, far fewer feature the necessary code to handle a backward move in time. These technical vulnerabilities ultimately led to the decision to cancel the leap second.