Civil time is occasionally adjusted by one second increments to ensure that the difference between a uniform time scale defined by atomic clocks does not differ from the Earth's rotational time by more than 0.9 seconds. Coordinated Universal Time (UTC), an atomic time, is the basis for civil time.

Historically, the second was defined in terms of the rotation of the Earth as 1/86,400 of a mean solar day.  In 1956, the International Committee for Weights and Measures, under the authority given it by the Tenth General Conference on Weights and Measures in 1954, defined the second in terms of the period of revolution of the Earth around the Sun for a particular epoch, because by then it had become recognized that the Earth's rotation was not sufficiently uniform as a standard of time.  The Earth's motion was described in Newcomb's Tables of the Sun, which provides a formula for the motion of the Sun at the epoch 1900 based on astronomical observations made during the eighteenth and nineteenth centuries.  The ephemeris second thus defined is

        the fraction 1/31,556,925.9747 of the tropical year for 1900 January 0 at12 hours ephemeris time.

This definition was ratified by the Eleventh General Conference on Weights and Measures in 1960.    Reference to the year 1900  does not mean that this is the epoch of a mean solar day of 86,400 seconds.  Rather, it is the epoch of the tropical year of 31,556,925.9747 seconds of ephemeris time.  Ephemeris Time (ET) was defined as the measure of time that brings the observed positions of the celestial bodies into accord with the Newtonian dynamical theory of motion.

Following several years of work, two astronomers at the U.S. Naval Observatory (USNO) and two astronomers at the National Physical Laboratory (Teddington, England) determined the relationship between the frequency of the cesium atom (the standard of time) and the ephemeris second. They determined the orbital motion of the Moon about the Earth, from which the apparent motion of the Sun could be inferred, in terms of time as  measured by an atomic clock.  As a result, in 1967 the Thirteenth General Conference on Weights and Measures defined the second of atomic time in the International System of Units  (SI) as

the duration of  9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.

The Sub-bureau for Rapid Service and Predictions  of Earth Orientation Parameters of the International Earth Rotation Service (IERS), located at the USNO, monitors the Earth's rotation. Part of its mission involves the determination of a time scale based on the current rate of the rotation of the Earth. UT1 is the non-uniform time based on the Earth's rotation.

The Earth is constantly undergoing a deceleration caused by the braking action of the tides. Through the use of ancient observations of eclipses, it is possible to determine the average deceleration of the Earth to be roughly 1.4 milliseconds per day per century. This deceleration  causes the Earth's rotational time to slow with respect to the atomic clock time. Thus, the definition of the ephemeris second embodied in Newcomb's motion of the Sun was implicitly equal to the average mean solar second over the eighteenth and nineteenth centuries.  Modern studies have indicated that the epoch at which the mean solar day was exactly 86,400 SI seconds was approximately 1820.  This is also the approximate mean epoch of the observations analyzed by Newcomb, ranging in date from 1750 to 1892, that resulted in the definition of the mean solar day on the scale of Ephemeris Time.  Before then, the mean solar day was shorter than 86,400 seconds and since then it has been longer than 86,400 seconds.

The length of the mean solar day has increased by roughly 2 milliseconds since it was exactly 86,400 seconds of atomic time about 1.88 centuries ago (i.e. the 188 year difference between 2008 and 1820).   That is, the length of the mean solar day is at present about 86,400.002 seconds instead of exactly 86,400 seconds.  Over the course of one year, the difference accumulates to almost one second, which is compensated by the insertion of a leap second into the scale of UTC with a current regularity of a little less than once per year.  Other factors also affect the Earth, some in unpredictable ways, so that it is necessary to monitor the Earth's rotation continuously.

In order to keep the cumulative difference in UT1-UTC less than 0.9 seconds, a leap second is added to the atomic time to decrease the difference between the two. This leap second can be either positive or negative depending on the Earth's rotation. Since the first leap second in 1972, all leap seconds have been positive and there were 23 leap seconds in the 34 years to January, 2006.   This pattern reflects the general slowing trend of the Earth due to tidal braking.

Confusion sometimes arises over the misconception that the regular insertion of leap seconds every few years indicates that the Earth should stop rotating within a few millennia. The confusion arises because some mistake leap seconds for a measure of the rate at which the Earth is slowing. The 1 second increments are, however, indications of the accumulated difference in time between the two systems.  (Also, it is important to note that the current difference in the length of day from 86,400 seconds is the accumulation over nearly two centuries, not just the previous year.)  As an example, the situation is similar to what would happen if a person owned a watch that lost 2 seconds per day. If it were set to a perfect clock today, the watch would be found to be slow by 2 seconds tomorrow. At the end of a month, the watch will be roughly a minute in error (30 days of 2 second error accumulated each day). The person would then find it convenient to reset the watch by one minute to have the correct time again.

This scenario is analogous to that encountered with the leap second. The difference is that instead of setting the clock that is running slow, we choose to set the clock that is keeping a uniform, precise time. The reason for this is that we can change the time on an atomic clock, while it is not possible to alter the Earth's rotational speed to match the atomic clocks! Currently the Earth runs slow at roughly 2 milliseconds per day. After 500 days, the difference between the Earth rotation time and the atomic time would be 1 second. Instead of allowing this to happen, a leap second is inserted to bring the two times closer together.

International Atomic Time (TAI)  is a statistical atomic time scale based on a large number of clocks operating at standards laboratories around the world that is maintained by  the Bureau International des Poids et Mesures; its unit interval is exactly one SI second at sea level. The origin of TAI is such that UT1-TAI is approximately 0 (zero) on January 1, 1958. TAI is not adjusted for leap seconds.  It is recommended by the BIPM that systems which cannot handle leapseconds use TAI instead.

Coordinated Universal Time (UTC) is defined by the CCIR Recommendation 460-4 (1986). It differs from TAI by the total number of leap seconds, so that UT1-UTC stays smaller than 0.9s in absolute value.   The decision to introduce a leap second in UTC is the responsibility of the International Earth Rotation Service (IERS). According to the CCIR Recommendation, first preference is given to the opportunities at the end of December and June, and second preference to those at the end of March and September. Since the system was introduced in 1972, only dates in June and December have been used.  TAI is expressed in terms of UTC by the relation TAI = UTC + dAT, where  dAT is the total algebraic sum of leap seconds.

The first leap second was introduced on June 30, 1972. The historical list of leap seconds can be found here.

The Global Positioning System (GPS) epoch is January 6, 1980 and is synchronized to UTC. GPS Time is NOT adjusted for leap seconds.

BEFORE THE 2016 LEAP SECOND: GPS-UTC IS 17 (GPS AHEAD OF UTC BY 17 SECONDS)
AFTER THE 2016 LEAP SECOND:  GPS-UTC WILL BECOME 18 (GPS AHEAD OF UTC BY 18 SECONDS)

As of June 30 2015, and until the leap second of December 31 2016
        TAI is ahead of UTC   by 36 seconds.
        TAI is ahead of GPS   by 19 seconds.
        GPS is ahead of UTC   by 17 seconds.

After December 2016,
        TAI is ahead of UTC   by 37 seconds.
        TAI is ahead of GPS   by 19 seconds.
        GPS is ahead of UTC   by 18 seconds.

Terrestrial Time (TT) is a uniform atomic time scale, whose unit is the SI second, that replaces Ephemeris Time and maintains continuity with it.  TT may be regarded as the time that would be kept by an ideal atomic clock on the geoid.  To convert a TT value to a prediction of UT1, it is necessary to know the difference dT = TT - UT1.  Values of dT are tabulated in the Astronomical Almanac.  For example, mathematical predictions of lunar and solar eclipses in the distant past and future depend sensitively on estimates of dT.  The computed path of a solar eclipse that occurred 2000 years ago would be in error by about 3 hours, or some 45 degrees in longitude to the west, on the assumption that the rate of rotation of the earth were uniform.  Conversely, records of well documented ancient eclipses, together with modern telescopic observations of occultations, Very Long Baseline Interferometry, satellite laser ranging,  lunar laser ranging, and other measurements correlated to atomic time scales since 1955, have provided the data on which long term trends and short term fluctuations have been derived.  Since dT was approximately 32.184 seconds at the origin of TAI in 1958, a practical realization of TT is TT = TAI + 32.184 seconds. Although this expression gives TT in terms of TAI, in practice TT is obtained from the relation TT = UTC + dAT + 32.184 seconds for a known value of UTC and a given number of leap seconds.
 
The U.S. Naval Observatory is charged with the responsibility for the precise determination and dissemination of time for the Department of Defense and maintains DoD’s Master Clock. The U.S. Naval Observatory, together with the National Institute of Standards and Technology (NIST), determines time for the United States. Modern electronic navigation and communications systems depend increasingly on the dissemination of precise time through such mechanisms as the Internet-based Network Time Protocol (NTP) and the satellite-based Global Positioning System (GPS).

The U.S. Naval Observatory is the largest single contributor (at approximately 30 percent of the weighted average) to the international timescale UTC, which is computed in Paris, France, at the International Bureau of Weights and Measures. This international prominence in atomic timekeeping is due to the large number of atomic clocks operated by the Observatory and the fidelity to which they are measured and maintained. Moreover, the U.S. Naval Observatory’s principal role in keeping track of changes in the “Earth clock” (i.e., Earth rotation) and its dissemination of this information as the Rapid Service/Prediction Center for the IERS attests to the fact that globally, as well as nationally, the U.S. Naval Observatory remains the leader in precise time. Information concerning the U.S. Naval Observatory, its mission, history, and programs is available from our World Wide Web site at www.usno.navy.mil/USNO.
 

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Time Service Dept., U.S. Naval Observatory, Washington, DC