From the "local true solar time" and the "Greenwich mean time" (G M T) to the "Universal time coordinated" (U T C; In a better English "coordinated universal time", but CUT is not a good abbreviation).

Up to the eighteenth century the "time", or preferably the civil time used in ordinary life, was determined by the daily motion of the Sun. It was midday, literally the middle of the day, when the Sun was at its highest position in the sky and then in the direction of either South or North.

Unfortunately this "local true solar time" is very irregular for two main reasons : the Earth axis has a large inclination, about 23.5°, and the Earth orbit is an ellipse and not a circle. It was then difficult to build clocks able to follow this time (even if today this irregularity would be no more an obstacle) and a more regular time was defined : the "local mean solar time" based on the daily revolution of stars with its periodicity of 23 h 56 mn 4.09966 s.

The difference "local true solar time" minus "local mean solar time" has an annual periodicity and varies between - 14 mn 18 s (February 11) and + 16 mn 24 s (November 3).

However with this local system each city has its own time and with the development of railways the confusion became so great that many countries decided to adopt as civil time the mean solar time of their capital.

The mean solar time of the Greenwich observatory, near London (UK), became the civil time of all Britain in the 1880's under the name of G C T, Greenwich civil time, later called G M T, Greenwich mean time.

In the large countries, U S A, Canada, a unique time was not considered as possible (although today China, India and also continental Europe from Spain to Poland have a unique hour) and the time-zone system was slowly organized.

The global time-zone system with its 24 zones was defined at the "international conference on hour" (Paris 1913). France was already at the G M T since March 9, 1911.

In 1925 the G M T was renamed "universal time" (U T or T U, temps universel, or WZ, Weltzeit).

The progress of time measurements have led to the following subdivisions.

T U zero or T U 0 is the original G M T defined by the daily revolutions of stars as seen from the Greenwich observatory.

T U 1 is a more regular time taking account of the small motions of the poles with respect to the Earth surface. It can be defined as an averaged mean time for many suitable observatories that would be regularly spaced on Earth.

T U 2 takes account of the main yearly variations of Earth rotation. These variations are related to the largest meteorological phenomena such as the monsoon.

T U 3 takes account of the main lunar effects either direct (attraction, modification of the vertical) or indirect (tides).

These four T U have only small differences

| T U 1 - T U 0 | < 30 milliseconds
| T U 2 - T U 1 | < 60 milliseconds
| T U 3 - T U 2 | < 4 milliseconds

The discovery of random variations of Earth rotation and of its very slow decrease related to tidal frictions has ruined the validity of Universal Time as a basis for a regular scale of time.

In 1955 the unit of time, the second, became defined by the motions of planets ("second of ephemeris") but this difficult and inconvenient definition was forsaken in 1967 for the accurate and convenient second defined by the atomic frequency standards.

A sufficiently large series of atomic clocks now gives the extremely regular and accurate "international atomic time" written T.A.I. (temps atomique international). Its full parallelism with the ephemeris time given by the motions of planets has been verified over 30 years with a relative accuracy of 10^-10

The ephemeris time is now called T.D.T. (temps dynamique terrestre, terrestrial dynamic time) and is henceforth defined by:

T D T = T A I + 32.184 seconds

the difference being there for historical reasons.

Our ordinary civil time cannot be based only on the T A I, it needs a relation to the Sun. We arrive thus to the "universal time coordinated" (U T C) that is standard since January 1, 1972.

The U.T.C. is defined by the following:

1. The difference ( T A I - U T C ) is always an integer number of seconds.
2. The difference | T U 1 - U T C | is always smaller than 0.9 second.
3. From time to time the director of the B I H (Bureau International de l'Heure ; Paris) decides to add an intercalar second in order to take account of the irregularities of the T U 1.
   The BIH has recently received a second name: International Earth Rotation Service (IERS).

These intercalar seconds always fall at the end of a semester : the last minute of time U T C has then 61 seconds.

Between 1972 and May 2001, there have been 22 intercalar seconds at the following dates:

The difference ( T A I - U T C ), that was 10 seconds in the first semester of 1972, reached in 2000 the value of 32 seconds. The universal time coordinated, U T C, has thus a mixed status. It has the extremely regular unit of time of the international atomic time, T A I, but is also related to the irregular Earth rotation and can thus, for instance, be used for the determination of longitudes.

For very accurate determinations the publications of the Bureau International de l'Heure allows to know the difference ( U T C - T U 1 ) at any time with an accuracy of one millisecond.

The U.T.C remains always very near to the old T U 1 and G M T. It plays now the role formerly played by the G M T, for instance:
U.S. Eastern Standard Time (E.S.T.) = U T C - 5 hours
U.S. Eastern Daylight savings Time (E.D.T.) = U T C - 4 hours.

The Greenwich mean time, GMT, has today only an historical interest. It has been abandoned since the thirties for successively the T U 1, the T U 2 and finally, in 1972, for the much more regular universal time coordinated, U T C, that must be used for all present use.

International abbreviation of U.T.C.:

We must write the U T C dates in the following way from the largest to the smallest units.

For instance March 25, 1995 at 6 hours 38 minutes 4.73 seconds U T C has the following international abbreviation:

1995-03-25-06-38-04.73 U T C

The corresponding day is of course 1995-03-25 and the corresponding hour is 1995-03-25-06 U T C.

LINKS:

• International Earth Rotation Service: hpiers.obspm.fr or www.iers.org.
• Time Server of the 'Bureau International des Poids et Mesures'
• The Official U.S. Time