Universal time (UT) is simply the number of hours, minutes, and seconds which have elapsed since midnight (when the Sun is at a longitude of 180°) in the Greenwich time zone.

Since the Earth's rotation is irregular at the 0.1 second level, a local approximation to universal time not corrected for polar motion is often used. This is called UT0, and also referred to as Greenwich mean time, abbreviated GMT. In UT0, 24 universal hours are defined to be a mean solar day.

The following table gives the conversions between universal time (UT) and standard and daylight saving time in the United States.

The actual universal time (denoted UT or UT1) is tied to the rotation of the Earth. Because the Earth's rotation rate is rather irregular and unpredictable at the 0.1 s level, Universal Time can only be deduced from observations of star transits. Once known, UT can be compared with known ephemeris time, and the difference can be derived. UT is always kept within 0.9 seconds of coordinated universal time (what WWV and other time broadcast services provide) by the insertion or deletion of leap seconds, usually at 23:59:59 UTC on either June 30 or December 31. The following table gives for 1990-2000 (Astronomical Almanac, p. K9), where indicates an extrapolated value.

Year		Year
1990.0	56.86	2000.0
1991.0	57.57	2001.0
1992.0	58.31	2002.0
1993.0	59.12	2003.0
1994.0	59.98	2004.0
1995.0	60.78
1996.0	61.63
1997.0	62.29
1998.0	62.97
1999.0

Barycentric Dynamical Time, Coordinated Universal Time, Daylight Saving Time, Ephemeris Time, Greenwich Mean Time, International Atomic Time, Standard Time, Time