The leap second proved so disruptive that timekeepers voted to abolish it by 2035

For more than half a century, the world’s official clocks have occasionally paused for a single extra second so that human timekeeping could stay in step with the slightly irregular spin of the planet. That fix, known as the leap second, caused enough software failures and scheduling headaches at major technology companies that the international body responsible for global measurement standards voted in 2022 to phase it out entirely by 2035. The decision closes a decades-long experiment in reconciling atomic precision with a planet that refuses to rotate at a perfectly constant speed.

Why Clocks Needed an Occasional Extra Second at All

Coordinated Universal Time, the time standard the world runs on, is generated by a global network of atomic clocks that tick with extraordinary consistency, far more precisely than Earth itself rotates. Earth’s spin is subject to tiny variations caused by factors including the pull of the moon, shifts in the planet’s molten core, and even the redistribution of mass from melting ice sheets, all of which can very slightly speed up or slow down the length of a day. Left unchecked, that mismatch between atomic time and solar time would eventually cause clocks to drift out of sync with the actual position of the sun in the sky.

To prevent that drift from accumulating, the International Earth Rotation and Reference Systems Service began inserting an extra second, a leap second, into Coordinated Universal Time whenever the gap between atomic time and Earth’s rotation approached roughly a full second. The first leap second was added in 1972, and dozens more followed over the subsequent decades, almost always inserted at the end of June or December, with the timing announced only a few months in advance based on the latest rotation measurements.

How a One-Second Adjustment Broke Computer Systems

While a single extra second sounds trivial, it proved surprisingly disruptive to the software systems that quietly run much of modern life. Many computer systems are built around the assumption that time moves forward in perfectly uniform increments, and a leap second violates that assumption by inserting a repeated or unusual timestamp into the middle of a day. Several major outages and glitches over the years were traced directly to leap second insertions, including problems that affected airline booking systems, cloud computing platforms, and widely used websites when the extra second was added, with one widely reported 2012 insertion knocking a number of popular online services offline for hours.

Large technology companies responded by developing workaround techniques such as spreading the extra second’s adjustment gradually across many hours rather than inserting it all at once, a method sometimes called a leap smear. Even with such workarounds, the unpredictability of exactly when a leap second would next be required, since it depends on Earth’s rotation rather than a fixed schedule, made it a recurring source of engineering risk that companies increasingly argued was no longer worth the trouble it caused. Financial markets and satellite navigation systems, both of which depend on extremely precise and consistent timestamps, were also flagged repeatedly as areas where an unplanned extra second carried outsized risk.

The 2022 Vote to Retire the Leap Second

In November 2022, representatives at the General Conference on Weights and Measures, the international body that oversees global measurement standards, voted to eliminate the leap second by no later than 2035. Under the resolution, no new leap seconds are meant to be inserted after that point, at least until Coordinated Universal Time is allowed to drift from solar time by a larger amount, likely close to a full minute, before officials decide on a new, less disruptive method for realigning the two.

The change effectively buys decades of breathing room, since the gap between atomic time and solar time grows only very gradually. Supporters of the move argued that letting the discrepancy build up to a full minute before intervening, rather than nudging it back into alignment every year or two with single-second insertions, would be far easier for software systems handling Coordinated Universal Time to manage predictably, since a change measured in minutes rather than seconds could be scheduled and tested well in advance.

What Happens as Earth’s Rotation Keeps Shifting

Ironically, in the years surrounding the decision to retire the leap second, some scientists who track Earth’s rotation began noting the opposite problem than the one that originally justified leap seconds. Instead of consistently slowing down and requiring extra seconds to be added, the planet’s rotation has at times sped up slightly, raising the theoretical possibility that a negative leap second, subtracting a second rather than adding one, could eventually be needed under the old system. No negative leap second has ever been implemented, and the 2035 phase-out plan is intended to avoid the need for either kind of adjustment for the foreseeable future.

Timekeeping authorities continue to monitor Earth’s rotation closely even as the leap second itself heads toward retirement, since the underlying physical reality, a planet whose spin never quite matches the steady tick of an atomic clock, has not gone away. What has changed is the timekeeping community’s appetite for correcting that mismatch with an unpredictable, disruptive single-second jolt inserted into the calendar every year or two, in favor of a slower, more predictable adjustment that gives software engineers years of advance notice instead of a few months.

This article was produced with the assistance of AI and reviewed by Morning Overview editors.

More from Morning Overview