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Leap Seconds Will Expire by 2035, Easing Time Trouble for Tech

An international time standards group concludes that fine-tuning clocks to match Earth's rotation is more trouble than it's worth.

A closeup of a mechanical watch shows stellar constellations
Leap seconds have been added to global timekeeping since 1972.
Vacheron Constantin

Leap seconds, used for a half century to synchronize atomic clocks with variations in the Earth's rotation, are being phased out. That's good news for tech giants that are worried about the adjustments' technical risks.

Timekeeping authorities from around the world voted Friday at a meeting of the Bureau International des Poids et Mesures, or BIPM, to stop using the temporal tweak.

"The ... introduction of leap seconds creates discontinuities that risk causing serious malfunctions in critical digital infrastructure," including satellite navigation systems, telecommunications and energy transmission, BIPM said of its reasoning for the move.

The change will take effect no later than 2035, though it's possible the group could phase it in sooner. The new policy is designed to last at least a century.

Timekeeping with this level of precision may sound like an arcane scientific domain, but it actually matters a lot in our digital age, in which computers must constantly track tasks and make sure actions take place in the right sequence. Ticking digital clocks are a foundation for everything we do online.

In August, Facebook pushed for an end to the leap second, warning the transition could have "devastating effect on the software relying on timers or schedulers." It's not alone: A committee associated with BIPM surveyed measurement, scientific and technology experts and found the same view.

It isn't an idle worry. The leap second change triggered a massive Reddit outage in 2012, as well as related problems at Mozilla, LinkedIn, Yelp and airline booking service Amadeus. In 2017, a leap second glitch at network infrastructure company Cloudflare knocked a fraction of customers' servers offline. Cloudflare's software, comparing two clocks, calculated that time had gone backward and couldn't properly handle that result.

Phasing out the leap second as late as 2035 leaves 13 years of possible tech trouble. But adopting the new policy has some complications, including Russian pressure for a delay. Russia's satellite navigation system — unlike those operated by the United States, Europe and China — factors in leap seconds.

Earlier leap seconds have been added to compensate for the Earth's slowing rotation, but there's evidence the rate is now speeding up. That would mean a leap second would have to be removed, and that's never been tried.

Though the leap second no longer will routinely be added when Earth time disagrees with Coordinated Universal Time, or UTC, the BIPM vote leaves the door open for adjustments. It recommended forming a policy for adjusting clocks at some as yet unspecified time gap.