On June 30, 2015 23:59:59 a leap second with be inserted to Coordinated Universal Time (UTC)

Leap seconds are a periodic intervention in order to keep a system’s time of day close to the mean solar time. Without this adjustment, time defined by Earth’s spin drifts away from atomic time. Since this system of correction was defined in 1972, 26 correction interventions have been conducted. The last correction dates from 3 years ago: June 30, 2012 23:59:59.

Understanding the leap second (source: RedHat KB Article)

The basic time for mostly all of the world’s local time zones is called Coordinated Universal Time, UTC, which is derived from a bunch of atomic clocks which are distributed in several countries all over the world. The rotation of the earth is not very constant and varies a bit over time, while decreasing the mean rotation speed slowly. This is the reason why so called leap seconds are inserted into the UTC time scale, they adjust process of the UTC time to the real earth rotation.

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