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, so leap seconds are added (UTC) to make sure this discrepancy does not get too large over time and to synchronize the time we use as much as possible with the Earth's rotation.
Leap seconds have been the subject of many debates, with civil time already offset by an hour or more from the local mean solar time in many places, due to time zones and, in some countries, 1-hour daylight saving time changes twice a year, it is argued that most people will not notice a small, very slow drift of civil time away from solar time if leap seconds are abandoned.
Complex timekeeping systems and software requires leap second to be added manually. Because the Earth's rotation is irregular, leap seconds do not follow a simple pattern so each leap second has to be programmed in separately after it is announced with only six months' notice. This can be a difficult and costly process, and it means that human error could effect timekeeping systems.
In 2012 when the previous adjustment took place Mozilla, Reddit, Foursquare, Yelp, LinkedIn, and StumbleUpon all reported crashes and there were problems with the Linux operating system and programs written in Java, while Google developed its own unique system of adding leap seconds.
Losing leap seconds would mean losing the link between the Sun and time, astronomers and earth scientists making measurements based on the Earth's rotation would have to compensate for the loss of leap seconds in their systems, which would also be a manual and difficult process.