Markets and tech companies braced for glitches as extra second introduced to allow atomic clocks to stay in sync with the Earth’s gradually slowing rotation

Time and tide wait for no man, the saying goes. But at midnight GMT on Tuesday clocks will pause momentarily as the entire planet gains a bonus second.

If you happen to be awake, and gazing at the dial on an atomic clock, it will read 23:59:60 before ticking forward to 00.00.00.

The addition of a “leap second” is designed to allow the Earth’s rotation, which is gradually slowing, to catch up with atomic clocks, keeping official time neatly in sync with night and day. But trade floors, tech companies and those in charge of the internet are bracing themselves for potentially calamitous computer glitches linked to the 61-second minute.

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“There are consequences [to] tinkering with time,” said Peter Whibberley, a senior scientist at the National Physical Laboratory, which is now responsible for defining Greenwich Mean Time. “Because leap seconds are only introduced sporadically it is difficult to implement them in computers and mistakes can cause systems to fail temporarily.”

Last time a leap second was added, on a weekend 2012, Mozilla, Reddit and LinkedIn all crashed. In Australia, more than 400 flights were grounded as the Qantas check-in system went down, requiring the job to be done manually.

This time is the first time since markets went electronic that the 61-second minute will occur during trading hours, adding to market nerves linked to the looming Greek deadline.

Dr Leon Lobo, business development manager at NPL, has been involved in preparations for this evening: “If everyone adds the second in the same way at the same time it shouldn’t cause problems. But if some apply it in a different way or at a slightly different time, you start to have discrepancies in the time that people have. That’s when things trip up.”

The need for leap seconds is, in a sense, because official timekeeping has become so precise. Atomic clocks are roughly a million times better at keeping time than the rotation of the Earth, which fluctuates day-to-day and in the long term is slowing down, due to a phenomenon known as “moon drag”.

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The moon’s gravity raises tidal bulges on the Earth’s surface. As the Earth rotates, the bulges move out of line with the moon, but they are continually being dragged backwards by the moon’s gravity, causing a frictional drag between the water and ocean floors and coastlines.

“The tidal bulges act like giant brake pads,” said Dr Marek Kukula, public astronomer at the Royal Observatory Greenwich. “Over millions of years they cause the Earth’s rotation to slow down.”

Without the correction, civil time would very slowly drift away from time based on the Earth’s rotation, meaning that about 800 years from now the sun would be at its highest point in the sky at 1pm rather than noon.

“We’re all uncomfortable with the idea of our clocks being out of sync with the night and day cycle - it’s a psychological thing,” said Kukula. “There’s a vague sense of disquiet about our official systems drifting out of time with the natural cycle. But when you stick an extra second into the time system, it does cause issues. A lot of computer systems don’t like it.”

Leap seconds have been added to the world’s computers around once a year since 1972 – this is the 26th - but this occasion could also be the last. The world’s time keepers are divided over the issue, with some nations arguing that the addition of rogue seconds has become a liability in a world with an increasing number of financial and communications systems reliant on ever greater precision in timing.

The US and France are pushing to abolish leap seconds, with Britain, Russia and China arguing that the technical challenges are manageable.

“We have always taken the Earth’s rotation as the ultimate reference for timekeeping, and astronomers and navigators still make use of it,” said Whibberley. “We shouldn’t break the link without carefully weighing the consequences.”

A final decision is due to be reached in November at a meeting of the International Telcommunications Union, the relevant UN body.