'Leap Second' Clocks In On June 30 - InformationWeek

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'Leap Second' Clocks In On June 30

It doesn't sound like a big deal, but every second counts in the Network Time Protocol upon which the Internet and all apps that synch with it are based.

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At the end of June, a rare event will occur. Due to a barely discernible slowing of the Earth's rotation, a leap second will be added to the Network Time Protocol (NTP) to keep it synchronized with the slowly lengthening solar day.

That might seem like a simple thing to do, but as John Engates, CTO at Rackspace, said when asked about keeping computer clocks synchronized: "Time gets complicated fast." Not everyone will add the leap second in the same way, or at same time. Some organizations, including Google, will do it their own way.

In fact, as you stick with us through this article, you may be longing for Morgan Freeman to step in and break it down for you, as he does for the Science Channel series Through the Wormhole. We can't give you Freeman, but we can give you all you need to know about why the leap second really matters to IT.

The original author of the NTP protocol, Prof. David Mills at the University of Delaware, set a direct and simple way to add the second: Count the last second of June 30 twice, using a special notation on the second count for the record.

Google has divined what it calls a "clever" way to do it, adding bits of a second throughout the day on June 30, so that there's no jarring last-second adjustment to clocks. It calls its method the "Google Smear."

(Image: Geralt via Pixabay)

(Image: Geralt via Pixabay)

"We have a clever way of handling leap seconds," wrote Google site reliability engineers Noah Maxwell and Michael Rothwell in a blog posted May 21. "Instead of repeating a second, we 'smear' away the extra second." Over a 20-hour period on June 30, Google adds a couple of milliseconds to each of its NTP servers' updates. By the end of the day, a full second has been added. As the NTP protocol and Google timekeepers enter the first second of July, their methods may differ, but they both agree on the time.

[ Does anybody really know what time it is? This guy does. Read NTP's Fate Hinges On Father Time. ]

Traditionalists -- some might call them purists -- like Harlan Stenn, chief maintainer of the Network Time Protocol, don't like the smear. "At noon on June 30, clocks (of smear implementers) will be off by a half second," he said in a February interview with InformationWeek about the intricacies of computer timekeeping. That's a lot in terms of precision time -- it might as well be a decade.

As the day wears on, processes based on precise timing -- such as the amount of time a valve opens to add a chemical to a mix -- will be off by more than a half-second if they relied on the Google smear. "What if you're getting radiation treatment? Do you want your radiation dose to be off by a half-second or more?" asked Stenn.

Why Every Second Really Counts

The last time a second needed to be added to the day was on June 30, 2012. For Qantas Airlines in Australia, it was a memorable event. Its systems, including flight reservations, went down for two hours as internal system clocks fell out of synch with external clocks. Prior to 2012, a second was added on Dec. 31, 2008, and also in 2005. The process started in 1972, and we'll have made a total of 36 additions by the end of the day on June 30, 2015.

There are agencies that are good at measuring the solar day, such as the US Naval Observatory in Washington and the Royal Observatory in Greenwich, UK. They dictate when the need to add a second arises. But no one supervises how the addition is made to all computer systems.

NTP does it in the way that it does because it's coordinating millions of computers based on the Posix standard, a 1989 relic that was meant to resolve differences between various Unix brands. Linux and Windows have since adopted Posix standards. Posix dictates that there are exactly 86,400 seconds in the day, every day, no more, no less. To simply add a second to June 30, and count it accurately in NTP, would throw the count permanently out of synch for all Posix-based computers relying on NTP time servers.

Next Page: 86,400 Seconds Every Day

 

Charles Babcock is an editor-at-large for InformationWeek and author of Management Strategies for the Cloud Revolution, a McGraw-Hill book. He is the former editor-in-chief of Digital News, former software editor of Computerworld and former technology editor of Interactive ... View Full Bio

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Charlie Babcock
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Charlie Babcock,
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6/17/2015 | 1:05:59 PM
Needed: A way to resolve time-keeping issues
Anyone think we could use a Court of Appeals for time issues? Of course, we don't have one. Remember, there are legal consequences when your medical device delivers a time stamp that differs from that of the NTP protocol.
Charlie Babcock
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Charlie Babcock,
User Rank: Author
6/18/2015 | 1:14:31 PM
The solar day makes sense to a lot of humanity, historically speaking
LinuxGuy, I have to disagree. Precise time-keeping originally had to do with navigating oceans in saling ships. Pendumlum based clocks for time-keeping didn't work, due to the rocking motion of the ship. And surely you're familiar with the difficulties of determining latitude and longitude using a sextant. You have to precisely capture the angle of the moon or a star to the horizon... oh nevermind. Noon and midnight need to be fixed points in the day against which other time processes can be aligned. To have different parties moving them around, depending on whether they wish to observe solar time or not makes no sense to most of humanity.
Charlie Babcock
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Charlie Babcock,
User Rank: Author
6/24/2015 | 2:22:09 PM
Forget lattitude, just use attitude
"Seriously, could this really be a problem that had to be corrected. LinuxGuy-VPK" If you detach time from the solar day, then you have to start making adjustments in longitude and navigation. Longitude zero or the prime meridian runs through Greenwich, England, and for centuries, travelers have gauged where they are in the world based on the amount of time they've traveled at what speed and maps using lattitude and longitude. If we ignore the solar day, then the prime meridian has to start migrating from Greenwich toward Paris at the rate of a football field a year. This isn't a problem for LinuxGuy, who would dispense with all that longitude and lattitude hocus pocus and just use attitude. (Tip of the hat to Rob Seaman, data engineer at the National Optical Astronomy Observatory, who pointed this out.)
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