Some basic information about the different time scales

Originally posted to sci.astro.fits on Dec 15, 1999, by Paul Schlyter.

TAI
International Atomic Time. Defined by about a dozen atomic clocks distributed worldwide. TAI-UT1 was approximately 0 on 1958 Jan 1.

UTC
Coordinated Universal Time. Differs from TAI by an integral number of seconds. When needed, leap seconds are introduced in UTC to keep the difference between UTC and UT less than 0.9 s. UTC was introduced in 1972.

UT
Universal time. Defined by the Earth's rotation, and determined by astronomical observations. This time scale is slightly irregular. There are several different definitions of UT, but the difference between them is always less than about 0.03 s. Usually one means UT2 when saying UT. UT2 is UT corrected for pole wandering, and seasonal variations in the Earth's rotational speed. We also have UT1 (not corrected for seasonal variations) and UT0 (not corrected at all).

ET
Ephemeris Time. Was used 1960-1983, and was replaced by TDT and TDB in 1984. For most purposes, ET up to 1983 Dec 31 and TDT from 1984 Jan 1 can be regarded as a continuous time-scale.

TDT
Terrestial Dynamical Time. Used as a time-scale of ephemerides from the Earth's surface. TDT = TAI + 32.184. Formerly called ET, Ephemeris Time.

TDB
Barycentric Dynamical Time. Used as a time-scale of ephemerides referred to the barycentre of the solar system. Differs from TDT by at most a few milliseconds.

TT
Terrestial Time. Used instead of TDT or TDB when the difference between them doesn't matter.

GMT
Greenwich Mean Time. It's ambiguous, and is now used (although not in astronomy) in the sense of UTC in addition to the earlier sense of UT. Prior to 1925, it was reckoned for astronomical purposes from Greenwich mean noon (12h UT) 
TDT = TAI+32.184s  ==>  UT-UTC = TAI-UTC - (TDT-UT) + 32.184s

Starting at    TAI-UTC   TDT-UT    UT-UTC
 
1972-01-01       +10     +42.23    -0.05
1972-07-01       +11     +42.80    +0.38
1973-01-01       +12     +43.37    +0.81
1973-07-01        "      +43.93    +0.25
1974-01-01       +13     +44.49    +0.69
1974-07-01        "      +44.99    +0.19
1975-01-01       +14     +45.48    +0.70
1975-07-01        "      +45.97    +0.21
1976-01-01       +15     +46.46    +0.72
1976-07-01        "      +46.99    +0.19
1977-01-01       +16     +47.52    +0.66
1977-07-01        "      +48.03    +0.15
1978-01-01       +17     +48.53    +0.65
1978-07-01        "      +49.06    +0.12
1979-01-01       +18     +49.59    +0.59
1979-07-01        "      +50.07    +0.11
1980-01-01       +19     +50.54    +0.64
1980-07-01        "      +50.96    +0.22
1981-01-01        "      +51.38    -0.20
1981-07-01       +20     +51.78    +0.40
1982-01-01        "      +52.17    +0.01
1982-07-01       +21     +52.57    +0.61
1983-01-01        "      +52.96    +0.22
1983-07-01       +22     +53.38    +0.80
1984-01-01        "      +53.79    +0.39
1984-07-01        "      +54.07    +0.11
1985-01-01        "      +54.34    -0.16
1985-07-01       +23     +54.61    +0.57
1986-01-01        "      +54.87    +0.31
1986-07-01        "      +55.10    +0.08
1987-01-01        "      +55.32    -0.14
1987-07-01        "      +55.57    -0.39
1988-01-01       +24     +55.82    +0.36
1988-07-01        "      +56.06    +0.12
1989-01-01        "      +56.30    -0.12
1989-07-01        "      +56.58    -0.40
1990-01-01       +25     +56.86    +0.32
1990-07-01        "      +57.22    -0.04
1991-01-01       +26     +57.57    +0.61
1991-07-01        "      +57.94    +0.24
1992-01-01        "      +58.31    -0.13
1992-07-01       +27     +58.72    +0.46
1993-01-01        "      +59.12    +0.06
1993-07-01       +28     +59.55    +0.63
1994-01-01        "      +59.98    +0.20
1994-07-01       +29     +60.38    +0.80
1995-01-01        "      +60.78    +0.40
1995-07-01        "      +61.20    -0.02
1996-01-01       +30     +61.63    +0.55
1996-07-01        "      +62.0     +0.2
1997-01-01        "      +62.4     -0.2
1997-07-01       +31     +62.8     +0.4
1998-01-01        "      +63.3     -0.1
1998-07-01        "      +63.7     -0.5
1999-01-01       +32     +64.1     +0.1

Earlier, one could get the latest status concerning leap seconds in UTC by sending e-mail to adsmail@tycho.usno.navy.mil with a Subject: line of 'leap' and no text. You would then receive in reply a list of past and provisional future leap seconds. However, this site seems to not have been updated since 1994. More up-to-date information can be obtained among these sites (thanks to Matthew.Healy@yale.edu for providing these URL's): 

Paul Schlyter,  Swedish Amateur Astronomer's Society (SAAF)
Grev Turegatan 40,  S-114 38 Stockholm,  SWEDEN
e-mail:  pausch@saaf.se    paul.schlyter@ausys.se   paul@inorbit.com
WWW:     http://hotel04.ausys.se/pausch    http://welcome.to/pausch

This is Michael again:

Other good sources of information are