Leap second threatened computers. Here’s what happened

Wednesday, July 1, 2015, 11:50 AM - When the clock struck midnight in London Tuesday, nervous website operators were watching closely.

That's when a single second was added at midnight UTC (Universal Coordinated Time), around 7 p.m. Eastern, and scientists were worried about what that would do to internal clocks for major websites and services.

As it happens, the impact of this 'leap second' wasn't great, but it was still an impact. According to the Telegraph, Twitter was confused by the extra second, not sure when Tweets were exactly sent out. Some Android users reported dates and notifications were malfunctioning, in some cases advancing July 1 several hours early.

More seriously, it looked like major websites like Netflix, Instagram, Pinterest and Amazon were down for as much as 40 minutes. Those, and thousands of other sites, rely on Amazon Web Services for Internet connectivity, and AWS initially told NBC the leap second was to blame.

However, the company later said the culprit was a connectivity issue with an external internet provider, not the leap second after all.

One major fear that also went unrealized was that the leap second would hurt stock markets. European markets, including London, were closed at the time of the adjustment, but markets in the U.S. and Asia did not report any problems.

Generally, CIO reports around 2,000 networks -- less than half a per cent of the global total -- stopped working for a certain amount of time, 50 per cent of those in Brazil.

So how does the 'leap second' work?

At just after 23:59:59 p.m. Greenwich Mean Time on Tuesday, June 30, most clocks paused — for just one second.

This inserted a "leap second" into the day, giving us just a moment where the unusual sight of 23:59:60 appears on official clock faces before they switch over to 00:00:00 on Wednesday, July 1.

The last time this happened, in 2012, several websites experienced problems due to the extra time, including FourSquare, Gawker, LinkedIn, Mozilla, Reddit and Yelp. In addition, over 400 Qantas Airlines flights were delayed when the Amadeus airline booking service went down for two hours, forcing staff at airports to switch to manual check-ins for passengers.

Why do we need a leap second?

Two of the ways that we measure time — the oscillation of cesium atoms in atomic clocks, and the rotation of the Earth — serve us very well in their day-to-day use. However, when it comes to the precision of these two time-keeping methods, they are very different from one another.

Cesium atoms oscillate so precisely, at exactly 9,192,631,770 times per second, that an atomic clock will keep accurate time for tens of millions of years before it will be off by even a second. In contrast, the time it takes for Earth to rotate once on its axis is far from precise, as this time gets longer due to the influence of the Moon.

"Earth's rotation is gradually slowing down a bit, so leap seconds are a way to account for that," said Daniel MacMillan, senior scientist of the Very Long Baseline Interferometry group at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

That's not all, though. Earth's rotation is also affected by random effects, such as powerful earthquakes and even large-scale weather events, which can add to or subtract from (on the order of microseconds) the length of the day.

These changes in Earth's rotation are fairly tiny in the short term, but it all adds up over time. Since the primary timekeeping method we use for our calendar — Coordinated Universal Time (or UTC) — uses a combination of atomic clocks (International Atomic Time or TAI) and the Earth's rotation (Universal Time or UT1), when the International Earth Rotation and Reference Systems (IERS) determines that these two time-keeping methods will be out of sync by 0.9 seconds, it adds a second to our clock. This is known as the leap second.

DID YOU KNOW? "Coordinated Universal Time" is initialized as UTC, rather than CUT, as an international compromise between the abbreviations in English and French (TUC, for Temps Universel Coordonné). "International Atomic Time" is abbreviated TAI for the French, Temps Atomique International, as it is the International Bureau of Weights and Measures, in France, that sets that standard for the world.

The very first leap seconds were added to the UTC clock in 1972, on both June 30 and December 31, and a total of 25 seconds have been added in the 42 years since, always on either June 30 or December 31. When they're added is far from regular though. It's decided whenever the two methods of keeping time are going to differ by more than 0.9 seconds. Up until 1998, they were added on almost a yearly basis. Since 1998, only three more have been added, in 2005, 2008, and the most recent one on June 30, 2012.

Synchronize your watches

Although the leap second does cause the rather novel "oddity" in what time our clocks display, not everyone agrees with their use.

One reason is that, unlike February 29 on a leap year, the addition of a leap second is anything but consistent. So, whenever one is applied, it is done manually. This has the potential to throw off various computer systems, especially those that rely on both UTC and very accurate time-keeping.

So, rather than continuing this practice of adding extra seconds onto UTC as a fix, scientists have proposed that we fix time itself — or, at least fix how we define time.

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One of the ways is to simply abolish the leap second, and toss out Coordinated Universal Time (UTC) along with it, thus switching us over to International Atomic Time (TAI) only.

What would this do? While we'd avoid some of the minor problems associated with adding the leap second, we'd have to get used to our clocks getting further and further away from solar time. Granted, over the nearly 50 years since the leap second was first instituted, the Earth's rotation has only slowed enough to add a grand total of 25 seconds (with the 26th added this June) - not to every day, mind you, but simply to the amount of time we've ticked off on our calendar. That's less than three one-hundredths of a percent of the length of a day. Had we simply switched over to the atomic standard immediately after the first atomic clock was built, would we have noticed that difference? For most of us, probably not.

However, this would affect some people in very important (and potentially expensive) ways, especially astronomers, given that many pieces of tracking hardware and computer systems have been built to incorporate leap seconds, and replacing that hardware (should the practice stop) would be difficult.

Astronomer Steve Allen, of the Lick Observatory and the University of California, Santa Cruz, has delved quite far into the subject on his website, in an effort to keep things as they are.

Not that there's much chance that anything is going to change with this, anytime soon. Proposals have been made and the issue has been tabled for discussion several times, at the very least in 2003, 2005 and most recently in 2012. However, any decision on the matter has been pushed further along for various reasons.

Editor's Note: A previous version of this article detailed the clock rolling over from 23:59:60 GMT June 30 to 00:00:01 GMT July 1, when it should have been from 23:59:60 GMT June 30 to 00:00:00 GMT July 1. This has been corrected. We apologize for any confusion and thank our readers for pointing out the error.

With files from Scott Sutherland and Daniel Martins

Sources: The Telegraph | CIO | The Guardian | NASDAQ | ICE | Google | Amazon | NASA