Tuesday’s Leap Second Explained

This Tuesday, a single second will be added to clocks around the world to help counter Earth’s rotation slowing down.

What is a “Leap Second”?

A leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time (UTC) in order to keep its time of day close to the mean solar time, or UT1. Without such a correction, time reckoned by Earth’s rotation drifts away from atomic time because of irregularities in the Earth’s rate of rotation. Since this system of correction was implemented in 1972, 25 such leap seconds have been inserted. The most recent one happened on June 30, 2012 at 23:59:60 UTC. A leap second, the 26th, will again be inserted at the end of June 30, 2015 at 23:59:60 UTC.

Humans can handle the additional second without even being aware of it, however computers are are another story and can get a little “confused”  where a path of time suddenly changes.

When a leap second was last added to the clock in 2012, during a weekend it wreaked some havoc online.

The leap second in 2012 caused Reddit, Foursquare, Yelp, LinkedIn, Gawker and StumbleUpon to be knocked offline entirely, as well as hundreds of flights to be delayed in Australia.

Many issues were caused by a bug in the Network Time Protocol used to keep Linux system clocks in sync. The flaw caused NTP to lock up some systems entirely, requiring a reboot before they could recover.

When the leap second comes around, it means the system clock sees an additional figure, like so:

2011-12-31 23.59.57
2011-12-31 23.59.58
2011-12-31 23.59.59
2011-12-31 23.59.60 <– leap second
2012-01-01 00.00.00
2012-01-01 00.00.01
2012-01-01 00.00.02

The second will be inserted into network time services at the exact same moment worldwide, on June 30th at 23:59:60 UTC.

This time around it’s critical that businesses are ready, with the leap second being added during a time when trading on stock markets is open.

Some businesses are ready for the leap second to be added, like Google and Amazon, which adjust server clocks gradually over a number of weeks so that it’s not a sudden change.

Others that rely on time-critical systems, like stock markets and utilities are nervous about it going wrong. A single second of downtime for a stock market means up to $4.6 million could be lost.

Linux systems specifically should be fine. The bug that affected them last time has since been resolved, along with other issues found in Java and other operating systems.

The leap second is mostly a headache for system administrators who need to ensure their services are highly available and need to plan how to handle the change. Hardware providers such as Cisco now provide detailed advice on how their hardware handles the leap second, but the side effects are unpredictable.

The Leap Second’s Future Demise?

Leap seconds might not be around for much longer with the International Telecommunications Union planning to vote on a proposal to eliminate the leap second in November 2015.

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