An additional “leap” second will be added on to the end of the day on June 30, 2015, resulting in an unusual 61-second minute.
This decision was made by the International Earth Rotation and Reference Systems Service (IERS), the global timekeeping body, in order to reconcile atomic clocks with the Earth's rotation around its own axis, which is gradually slowing down, although very slowly.
Two components are used to determine time. Coordinated Universal Time (UTC) utilizes 200 precise atomic clocks to provide the correct speed at which our clocks should tick and Astronomical Time (aka Universal Time) is based on the Earth’s rotation and used to compare UTC with the actual length of a day on Earth.
Because, unlike the Earth-rotation based measure, atomic clocks do not have any inconsistency we have to add a second every so often to co-ordinate the two; since 1972, a total of 25 “leap” seconds have been added with the last one taking place at 23:59:60 UTC on June 30, 2012.
So, instead of resetting to zero after reaching the 59 second mark at midnight on June 30 digital clocks tied to the IERS will instead read 23 hours, 59 minutes, and 60 seconds. You might want to write down this statistic for the next time you have a barstool trivia bet or discussion: days on which a leap second is inserted have 86,401 instead of the usual 86,400 seconds.