After decades of discussion, quibbling and worrying about clocks in computer systems, the world’s official timekeepers have voted to replace what many viewed as a tiresome correction: the leap second. It’s time, they say, for the leap hour.
Delegates from 69 countries cast their votes on Thursday at a conference in France of the International Bureau of Weights and Measures, which ensures the world sticks to the same standards of time, weight, distance and more.
The decision to adopt the leap hour from next year aims to reduce the risk of computers and other technical systems tripping over as their software grapples with minutes lasting 61 seconds – and, worse, 59 seconds – instead of a round 60.
“After 30 years of debate, the leap second is making way for the leap hour,” said Dr Setnam Shemar, principal scientist in the time and frequency department at the UK’s National Physical Laboratory. “This will reduce the risks to the systems and national infrastructure that depend on accurate time while still preserving the fundamental link between our global time system and the rotation of the Earth.”
The vote at Versailles means that coordinated universal time (UTC) can drift up to an hour from astronomical time before a leap hour is needed, but that is unlikely to happen for thousands of years.
“We already have daylight saving where we change by an hour and no one gets bothered about that, apart from losing sleep or getting an extra hour’s sleep,” said Shemar, who sat on the international timekeeping expert committee that proposed the leap hour.
The more familiar leap year was introduced more than 2,000 years ago to ensure that the calendar year did not fall out of step with the time that the Earth takes to circle the sun. Adding a day to the end of February every four or so years does the job, because Earth orbits the sun every 365 and a quarter days.
Leap seconds are a much more modern invention. In 1955, Louis Essen and Jack Parry at the National Physical Laboratory built the world’s first practical, high-precision caesium atomic clock. The device revolutionised timekeeping, and in 1967 the astronomical definition of a second, based on the rotation of the Earth, was abandoned. Ever since, the formal definition of one second has been 9,192,631,770 vibrations of the caesium atom.
Atomic clocks are exceptionally stable and precise. Their timekeeping has become critical for modern computer systems, communications, satellites, trading systems and electricity grids. But the Earth’s rotation is far less reliable. Its spin is affected by the planet’s molten core, the shift of the oceans, global jet streams and other natural forces.
Broadly, Earth’s rotation is slowing down: when an asteroid doomed the dinosaurs, a day lasted about 23 and a half hours. Because of this, officials have added 27 leap seconds since 1972 to ensure UTC is aligned with astronomical time.
The strategy endured for more than 50 years until scientists spotted a fresh reason to fear technological mayhem. In the past decade, Earth’s rotation has speeded up, meaning officials might have needed to add the first negative leap second, equivalent to lopping a second off UTC. The 30% probability of a negative leap second in the next decade brought the Versailles vote forward.
“Leap seconds can be tricky enough to deal with,” said Shemar. Telecommunications, electricity grids and navigation systems use high-accuracy timing, he explained, but software updates can apply leap seconds at the wrong time, knocking networks out of sync.
“After the leap second applied in 2012, there were some computer systems whose operating systems couldn’t cope with 61 seconds in a minute. The servers froze. They seized up. Systems went offline. And those are the sorts of failures that can happen with positive leap seconds,” Shemar said.
Among the crashed websites were Reddit and LinkedIn. Separately, Qantas flights were hit with delays. Negative leap seconds come with even greater risks. “There could be software out there that hasn’t been tested on negative leap seconds because we’ve never had to apply one before,” Shemar said.
To test all the computer systems that talk to one another around the world would cost companies a fortune. It was this concern that swung the vote.
Historically, the UK has grumbled about canning leap seconds because of the link with Greenwich mean time. Russia has also opposed the move, citing concerns of the impact on its Glonass satellite navigation system.
The IERS Earth Orientation Centre in Paris tracks changes in the Earth’s rotation and other movements that feed into UTC. The centre also issues notices when leap seconds must be added.
Dr Christian Bizouard, the director of the centre, approved of the vote, but said it passed the decision to people far in the future. “It’s kind of a philosophical discussion, because if a leap hour happens, it won’t be for a very, very long time,” he said. “What we are doing is pushing back the decision.”
