Time is very important to most of us. We have day planners and clocks to help us manage our daily lives. We also expect, due to our daily experiences, the sun to be near its highest position in the sky around noon and the stars to be at a certain place at midnight. Another way of saying this is: we expect our clocks to be synchronous with the time scale defined by the Earth's rotation. This astronomical time scale is defined as Universal Time (UT1).
The modern civil time that we are familiar with is based on the time zone we live in and the definition of the International System of Units (SI) second. However, this SI second has a uniform rate and does not take into account that we live on a rotating planet that is slowing down. This deceleration of the Earth's rotation is primarily the result of tidal friction between the Earth and its Moon and the Sun, and, to a lesser extent, deglaciation, post-glacial rebound, and the other planets. Therefore, our clocks have to be adjusted by the use of leap seconds, just as the leap year is used to correct the calendar. It is these adjustments that prevent noon from drifting closer to sunrise. This time scale is called Coordinated Universal Time (UTC) and is the basis of civil time.
UTC is an atomic scale that agrees in rate with international atomic time but differs by an integral number of seconds. It is the addition (or removal) of leap seconds that keep this time scale, UTC, "coordinated" with our planet's rotation and UT1. By international agreement, the differences between UT1 and UTC must be maintained to be less than one second. If it is determined by the International Earth Rotation and Reference Systems Service (IERS) that the difference between UT1 and UTC will become larger that one second, a leap second is added (or removed) from UTC to keep the difference less than one second. This agreement also indicates that the addition (or removal) of a leap second can only take place at four times during the year: December 31, June 30, March 31, or September 30.
A leap second is added into the UTC time scale by introducing a second into the scale after 23 hours (h), 59 minutes (m), and 59 seconds (s) of the day of the leap second and before 0 h, 0 m, and 0 s of the next day. The removal of a second from the time scale has never been done before. However, if it were needed the 59 s of 23 h and 59 m would be dropped.
A good way of thinking about the concept of adding a leap second is that we are adding a second into UTC to allow the Earth to catch up with our clocks (i.e., time scale). The technical process of actually adding the leap second into the time scale is just as simple. Normally, when we come to the end of the day we keep track of time by counting:
Day 365 23 h 59 m 58 s Day 365 23 h 59 m 59 s Day 001 00 h 00 m 00 s.
However, for adding a leap second, such as the leap second for this New Year's Eve, we will keep track of time by counting:
Day 365 23 h 59 m 58 s Day 365 23 h 59 m 59 s Day 365 23 h 59 m 60 s Day 001 00 h 00 m 00 s.
Thus, a leap second simply keeps our clock's time in sync with our Earth's rotation.