Universal Time

Today civil clock times throughout the world are based on a time scale known as Coordinated Universal Time, or UTC. UTC is often confused with GMT, Greenwich Mean Time, because times in the two scales are similar, but it is not at all the same. UTC is based on:

TAI, International Atomic Time, which is calculated statistically by the BIPM from results of atomic clocks maintained by various nations all over the world.
leap seconds, which can be added or subtracted. Leap seconds are preferably added at the end of December or June, with the ends of March and September as second choices. In fact, between 1972 and 2001 no seconds were subtracted, but 22 were added. (For a table showing when leap seconds have been added, see www.bldrdoc.gov/timefreq/pubs/bulletin/leapsecond.htm ) The need for a leap second is determined by the International Earth Rotation Service. The purpose of adding the leap seconds is to make the scale more closely track the Earth’s rotation, so that at 12:00 UTC the (mean) sun will be crossing the meridian at 0° of longitude (Greenwich, England).

History

In 1928, the International Astronomical Union (IAU) decided that the time scale used in almanacs should be called UT, Universal Time. The UT scale was based on the rotation of the Earth and was good for celestial navigation.

In 1935, the IAU dropped Greenwich Civil Time, replacing it with TU (Temps Universel, for French speakers), UT (Universal Time, for English speakers) and WZ (Welt Zeit, for German speakers).

Later the IAU decided that as of 1 January 1956, there would be three versions of Universal Time (the CGPM adopted these in 1960). In all the day begins at midnight.

UT0: Mean solar time at the prime meridian, ignoring the position of the Earth’s poles at that moment. For the middle latitudes, UT0 differs from UT1 by only a few hundredths of a second.
UT1: UT0 corrected for observed effects of the wandering pole (maximum of about 0.035 seconds). This time was used for celestial navigation.
After 1 January 1984, UT1 was defined in terms of Greenwich Mean Sidereal Time, so that the GMST1 time of midnight UT1 is 24110.54841 seconds + 8640184.812877T_u seconds + 0.093104T_u² seconds − 6.2 × 10⁻⁶T_u³ seconds,
where T_u is the number of days of Universal Time since or before noon on 1 January, 2000, divided by 36525. The number of days will always have the form whole number + 0.5, since Julian days begin at noon. 36525 is what is called a Julian century, the number of days in 100 years.
UT2: UT0 corrected for polar motion (max 0.035 seconds) and also variations in the Earth’s rotation rate (maximum of 0.035 seconds). Up to 1972, the time signals broadcast by most national services were based on UT2. (Transactions of the International Astronomical Union, x (1960) page 489.)

Two additional versions exist:

UT1R: UT1 corrected for zonal tidal effects. The difference is less than 2 milliseconds.
UT1R′: UT1 corrected for the effects of all tides.

Meanwhile, atomic clocks had been developed together with a new time scale, International Atomic Time (TAI), based upon them. TAI is completely independent of the Earth’s rotation.

In 1965, the Bureau International de l’Heure (BIH) developed a UT-like time scale from its atomic time scale, A3, and called it "UTC." In 1967, at the IAU’s 13th General Assembly, the Commissions on Ephemerides and Time Scales approved the name “Coordinated Universal Time (UTC).”

The Consultative Committee on Radiocommunications of the International Telecommunications Union developed a scale for broadcast time signals based on TAI. This new scale was named Coordinated Universal Time (UTC). It was defined by saying that at the beginning of 1 January 1972, UTC would be exactly 10 seconds behind TAI.

In order for UTC to match the Earth’s rotation, some compensation for the irregularities in that rotation must be made. Without corrections, the sun at noon would gradually drift from being overhead. Prior to 1972, broadcast time services had kept in step with the Earth’s rotation by subtly varying the frequency they broadcast. For UTC, the frequency was to be held constant. Corrections are made only by adding or subtracting a whole number of seconds, to prevent UTC from differing by more than 0.7, and later 0.9, seconds from TAI. The Bureau Internationale des Heures announces when a leap second is to be added or subtracted from UTC. (Since the Earth’s rotation is slowing, it is usually necessary to add a second.) The change is usually made at the end of June or December, but may also be made at the end of March or September.

In 1975 the CGPM endorsed UTC and recommended that clock time be based on it.

CCIR Recommendation 460-4 (1986)

The definition of Coordinated Universal Time.

The resistance to leap seconds

The unpredictable addition of leap-seconds, however, creates a problem for computer timekeeping, especially Global Positioning Systems, resulting in a call for broadcasting some form of atomic time, not UTC. To accomplish this, a new, leap-second free scale was proposed at a conference in Torino, Italy in May, 2003. The proposed scale, TI (Temps International, or International Time) would always be TAI minus 32 seconds (so it is expected to be equal to UTC in the year 2005). The proposal is controversial, see the sites below.

www.ien.it/luc/cesio/itu/arias_2.pdf

www.ien.it/luc/cesio/itu/annex_a.pdf

www.cl.cam.ac.uk/~mgk25/time/leap/

www.ucolick.org/~sla/leapsecs/

R. A. Nelson, D. D. McCarthy, S. Malys, J. Levine, B. Guinot, H. F. Fliegel, R. L. Beard and T. R. Bartholomew.
The leap second: its history and possible future.
Metrologia, volume 38, pages 509–529 (2001).