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.

Why this extra second? It exists because the rotation of the Earth on its axis, which determines the passing of days and nights, slows down over a long period, mainly as a consequence of Moon-Sun attraction effects. In addition, the Earth is affected by its internal (core, mantle) and external (atmosphere, oceans) constituents. Nowadays, though, time is measured largely by 250 atomic clocks belonging to several countries, which function by measuring the transition of energy levels in an atom. Together these clocks are used to calculate UTC, and as this time measuring mechanism is independent of the Earth periodically the two must be brought into alignment with a leap second. In addition, we have to consider that the length of the day is nowadays 2 ms longer than in the year 1820. Not surprisingly then, the Earth’s rotation slowly gets out of synchronization with UTC.

So what does it mean?

The leap second will be inserted at the same moment all over the world. The effective local time depend on the time offset from UTC. For example: if the timezone is UTC +3h then the leap second will be inserted at 23:59:59 + 3 hours.

The insertion is illustrated as follows:
2015-06-30 23.59.58
2015-06-30 23.59.59
2015-06-30 23.59.60 <-- leap second
2015-07-01 00.00.00
2015-07-01 00.00.01

What’s the impact?

Not all systems are capable to handle the leapdiag second and could crash the kernel. Ensure your system is up-to-date and running a kernel which is not affected by the adjustment.

Some additional info:

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