In 1972, Coordinated Universal Time (UTC) was established and this standard is used to set most of the world’s wristwatches and computers. This standard says that, at 11:59 pm and 60 seconds on June 30, 2015, there will be an extra second in this year. You probably won’t notice it, but computers definitely do. Unix-based systems have a particularly difficult time with leap seconds as they unvaryingly define a day as 84,600 seconds. The last time a leap second happened in 2012, Unix-based systems at Reddit, Yelp, and Foursquare all experienced. Wired.com has an interesting article about attempts to prevent problems before they happen.
You can read the complete Wired article here.