At 23:59:60 UTC on June 30, 2015, a Leap Second will be added, making this day last for 86401 seconds instead of the usual 86400. This practice started in 1972 and since then a total of 25 leap seconds were added to the UTC time, last one in 2012. Correction of each UTC leap second is usually decided about six months in advance by the International Earth Rotation and Reference Systems Service (IERS).
UTC, or Coordinated Universal Time is a compromise between Greenwich Mean Time, GMT based on the Earth's rotation and atomic clock time. In order for the time kept by atomic clocks to keep in step with GMT within 0.9 seconds, leap seconds are added or subtracted from UTC. Atomic clocks are extremely precise with deviation of about one second in 20 million years, defining one second as the interval of time it takes to complete 9,192,631,770 oscillations of the cesium 133 atom. In contrast, speed of the Earth's rotation differs from day to day and from year to year...
