World Timekeepers Decide to Kill the Leap Second

Human timekeeping rests on two main clocks, and they don’t always run at the same speed. For decades, the solution was the leap second, a minor adjustment added to international time to keep atomic accuracy aligned with the Earth’s natural motion. But on Thursday, the General Conference on Weights and Measures, or the CGPM (abbreviated from its French name), voted to abolish the leap second, closing a long-standing chapter in global time sync.

Most modern clocks rely on Coordinated Universal Time, a hybrid system caught between atomic time, which flows at a near-perfectly constant rate, and astronomical time, which fluctuates as the Earth’s rotation speeds up and slows down. To bridge the gap, UTC periodically inserted a leap second to keep pace with the planet. But reconciling physical planetary shifts with precise digital systems ultimately created more technical disruptions than it solved.

Breaking time

Computers are sensitive when it comes to timekeeping. The last few times we added a leap second to UTC — in 2012, 2015 and 2016 – some computer systems couldn’t handle it. Of the three instances, 2012 was the worst, as adding a leap second caused issues that crashed websites and software platforms, such as Reddit, LinkedIn and Mozilla.

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The speed of Earth’s rotation experiences small changes all the time. While we can’t predict exactly how fast Earth will rotate in the future, the conference wrote that there’s a 30% chance we’ll have to add a negative leap second by 2035 to keep UTC aligned with rotational time. 

If adding a regular leap second was trouble, imagine what adding a negative leap second could do. The conference doesn’t want to risk finding out. So, potential future computer anomalies caused by UTC changes are a major reason why the leap second is going away. 

“Industrial users and the synchronization needs of national critical infrastructures, for example, transportation, energy distribution, telecommunication, and space projects, require UTC to be continuous without any future adjustment,” the group’s resolution reads.

Moving forward

The official death of the leap second will be May 20, 2027, when continuous UTC becomes effective. A continuous UTC means that no more leap seconds, negative or otherwise, will be added to UTC going forward. This also means that UTC and rotational time will begin to drift apart. 

The CGPM noted that allowing time measurements to differ by up to an hour will keep UTC stable for several centuries.

This means in the future, when it’s noon, the sun might not be at its highest in the sky. But, considering we’ve only added 27 leap seconds since the concept was introduced in 1972, any changes that occur within our lifetimes will probably be very small. 

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Julian is a contributor and former staff writer at CNET. See full bio