Anyone who thinks that a date can’t wreak havoc in IT hasn’t tried to set the date of their iPhone to January 1, 1970 lately. But there’s an even more common calendar conundrum that is happening today: It’s Leap Year Day.

Every four years, an extra day is added to the calendar in February to make clocks and days come out even. “We have the orbit of the Earth to thank for the leap year,” explains Toyin Owoseje in the International Business Times. “As it takes the earth takes 365.2422 days to orbit the sun, and the Gregorian calendar only has 365 days, an extra day is added to help synchronize the calendar year with the solar year.”

Computers don’t always deal with it well. After its Azure cloud service had outages during the 2012 leap year, Microsoft recently put up an advisory on its website to software developers  to warn of potential code issues this leap year. Incidentally, they could also crop up on December 31, which will be the 366th day of the year.

To make things particularly complicated we don’t add an extra day to February in years that end in 00, unless the year is divisible by 400, in which case we do—which is why we had a leap year day in 2000 when we didn’t in 1900. Nor will we in 2100. (We also didn’t have a leap year in 2010, which was apparently a surprise to some Playstation 3s.) And, it turns out, Microsoft Excel still thinks 1900 is a leap year—because, at this point, fixing that bug could break so many other things.

That’s all complicated enough, but it gets worse. Last year wasn’t a leap year, but it contained a leap second, a literal extra second that’s occasionally added to the calendar to keep the atomic clock synchronized with the Earth’s rotation. Because they depend on measurements of the Earth’s rotation, which varies unpredictably, leap seconds occur at irregular intervals, Peter Whibberley, senior research scientist at the UK’s National Physical Laboratory explains to the BBC.

So on June 30, 2015, the last minute had 61 seconds in it. This has happened 27 times since the leap second was introduced in 1972, most recently in 2012.

You might think adding just a single second to the clock wouldn’t cause a problem, but it does. Altogether, some 2,000 networks crashed during the 2015 leap second, according to Jeremy Kirk in PC World.

It’s all because computers are getting so darn efficient. “Synchronizing traffic signals requires precision better than a minute,” writes David Yanofsky in Quartz. “Telephones, smartphones, the internet, and GPS synchronize at a precision of less than a second. The NASDAQ recently announced that from [October 1, 2015] it will increase the precision of its timestamps to include nanoseconds.”

The biggest problem, of course, is that the leap second can’t be planned for. Calculating a leap year day is convoluted, but at least there’s a formula; leap seconds are simply added when the International Earth Rotation and Reference Systems Service says we need one, and the world gets six months to prepare. As if IT didn’t have enough to do. Moreover, not everyone implements the leap second the same way, and therein lies problems.

“When a new leap second rolls around, things break,” writes Robinson Meyer in the Atlantic. “Reddit, LinkedIn, and Yelp all suffered issues related to the last leap second in 2012. And, more seriously, computer booking systems used by Qantas Airlines all struggled, delaying flights by hours.”

In fact, the leap second caused so much trouble that in 2012, and again last fall, the International Telecommunication Union (ITU), an arm of the United Nations, considered abolishing the leap second altogether. And it was a big deal—the 2012 meeting attracted 700 people.

Abolishing the leap second is a matter of contention, though. “If the leap second is stamped out, the astronomical definition of time will diverge from what is dictated by the atomic clocks, about a couple of thousandths of a second a day, growing to a minute over the course of a century,” writes Kenneth Chang in the New York Times. “Someday—thousands of years from now—noon will strike at sunrise instead of when the sun is overhead.”

Ultimately, delegates to the World Radio Conference, which makes the ruling, decided to put off the decision—not until 2023, writes Peter Sayer in PC World. Responsibility for the decision is also being moved from the ITU to another organization, most likely the General Conference on Weights and Measures, which already defines other scientific units such as the second, writes Elizabeth Gibney in Nature.

So IT staff will again be at their mercy when scientists decide the world needs to add another second to the clock. On the other hand, at least this year you’ve got an extra day to plan.

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