Time will stop on Tuesday, but only for a second. The world’s timekeepers will be adding a sliver of time—a leap second—to our clocks.
Just as leap years keep our calendars lined up with Earth’s revolution around the sun, leap seconds adjust for Earth’s rotation. This kind of fine-tuning wasn’t much of an issue before the invention of atomic clocks, whose ticks are defined by the cycling of atoms. Cesium-based clocks, one kind of atomic clock, measure the passage of time much more precisely than those based on the rotation of our planet, so adding a leap second allows astronomical time to catch up to atomic time.
It’s something time scientists have done since 1972, when the first leap second was instituted. In fact, researchers inserted 10 extra seconds into the world’s clocks because astronomical time and atomic time were off by so much.
“Synchronizing the planet is important for things that we take for granted, like navigation and email,” says Andrew Johnston, a geographer at Smithsonian’s National Air and Space Museum in Washington D.C. However, there are some time experts who argue against having a leap second, partly because of all the headaches they create every time they’re added.
Most of us won’t notice the addition, which happens at 23:59:59 coordinated universal time (UTC), or 7:59 p.m. ET, unless we deal in timescales shorter than a second, or if we use a computer program that crashes because it can’t handle the leap second. It’s happened before: The 2012 leap second brought down Reddit, Gawker Media, and Mozilla.
“It’s a major interruption mostly because there are a lot of systems that aren’t prepared to handle the leap second correctly,” says Judah Levine, a physicist at the National Institute of Standards and Technology (NIST) in Boulder, Colorado. Leap seconds occur irregularly, which makes it hard for programmers to test their fixes, he explains. source : http://news.nationalgeographic.com/2015/06/150629-leap-second-atomic-astronomical-time-earth-rotation-physics/