Leap Seconds Abolished: What the End of Leap Time Means for Global Timekeeping

Leap seconds could be abolished as early as October, potentially ending a timekeeping practice that has been used every few years since 1972. The adjustment was already scheduled to stop by 2035, but international authorities are considering ending it sooner because the next correction could be a negative leap second. Instead of adding one second to the world’s clocks, timekeepers might have to remove one—a change that has never occurred and could create significant risks for computer systems and critical infrastructure.

To prevent future disruptions, the world’s timekeeping authorities are proposing a new system based on “leap times.” Under the plan, clocks would be adjusted only when the difference between atomic time and Earth’s rotation reaches 3,600 seconds, or one hour. Members of the General Conference on Weights and Measures (CGPM) are scheduled to vote on the proposal during its 28th session near Paris from 13 to 15 October.

Since the 1970s, an additional second has been added to Coordinated Universal Time (UTC) 27 times. Leap seconds help keep atomic-clock time aligned with the planet’s naturally variable rotation, which has traditionally been used to define the length of a day.

However, leap seconds can cause serious problems for computer networks and digital services that depend on precise time synchronization. The affected systems include power grids, telecommunications networks, financial markets, satellite services and internet infrastructure.

One challenge is that organizations use different methods to manage a leap second. Google, for example, gradually spreads the extra second across the day, while other companies insert a full second into a specific minute. Some financial exchanges alter trading schedules so markets do not run for an additional 61 seconds. “It creates a discontinuity in a system that is supposed to be continuous,” says Georgette McDonald, director of the Metrology Research Center in Halifax, Canada.

Why a negative leap second could be dangerous

Future time corrections could be even more difficult to manage. Leap seconds are required because Earth’s rotation is not constant. Over millions of years, friction caused by the Moon’s gravitational pull has gradually slowed the planet’s rotation. Over shorter periods, however, Earth’s rotation can speed up or slow down. Researchers have observed an acceleration since around 2015, possibly linked to changes in the movement of material within Earth’s liquid core.

Scientists who monitor Earth’s rotation estimate that there is about a 30% chance that a negative leap second will be needed by 2035. In that situation, UTC would need to skip a second to remain aligned with astronomical time. Because no negative leap second has ever been implemented, the consequences for software, databases and critical infrastructure are difficult to predict. The International Bureau of Weights and Measures has warned that such an adjustment could disrupt essential services and be expensive to manage. “There’s a lot of anxiety about what kind of equipment failure is going to happen,” McDonald says.

Adjusting clocks only when the difference reaches one hour could make global timekeeping much easier, McDonald says. Such a correction would not be needed for hundreds or possibly thousands of years. The gradual separation between solar time and atomic time might also be less important than many people expect. Leap times could be simpler to implement than leap seconds because many countries already adjust clocks by one hour for daylight saving time. “From an implementation perspective, people are more familiar with hourly changes,” McDonald says.

Although a leap-time system could still create technical challenges, the technologies and financial systems of the distant future are likely to be very different from those used today. “I may not have thought the same way back then as I do now,” McDonald adds.

Upgrading the global timekeeping system