The rotation of the earth is slowing down and so we add a second to our clocks every once in a while.  The last time this happened was in 2008.  Another will be added this June 30.

A leap second is time added or subtracted to the atomic clock.

A leap second is a second, as measured by an atomic clock, added to or subtracted from Coordinated Universal Time (UTC) to make it agree with astronomical time to within 0.9 second. It compensates for the slowing in the Earth’s rotation and is added during the end of June or December. It is important to look at how seconds are used in relation to modern time keeping to gain an understanding of the concept of the leap second and why it is used.

The International Earth Rotation and Reference System Service (IERS) observes the Earth’s rotation and nearly six months in advance (January and July) a “Bulletin C” message is sent out, which reports whether or not to add a leap second in the end of June and December.

IERS schedules a leap second as needed to keep the time difference between atomic clocks and Earth’s rotation to below 0.9 seconds.

Seems pretty simple.  But somehow it has become controversial.  According to the New York Times

Opponents of leap seconds, led by the United States, say the sporadic addition of these timekeeping hiccups is a potential nightmare for computer networks that depend on precise time to coordinate communications.

But nations like Britain that wish to keep the current system say that eliminating leap seconds might create bigger problems.

They also oppose the uncoupling of time from the notion that the length of a day is tied to the motion of Earth and Sun. Because Earth’s rotation is gradually slowing, days are lengthening. Without leap seconds, noon on the clock would slide earlier and earlier in the morning.

This is a fight between nature and technology.  The decision about adding another leap second has been put off for three years but the leap second will be still be added in June.  This is how this will happen.