Difference Between Utc And Gmt

GMT stands for ‘Greenwich Meridian/Mean Time’. It's a time zone which nearly every country uses to set their local time. On the other hand UTC stands for ‘Universal Coordinate Time’. It's not a time zone but it is a time standard that is the basis for civil times and time zones throughout the world. World time zones have a positive or negative offset computed from Coordinated Universal Time (UTC), or Greenwich Mean Time (GMT). UTC has uniform seconds defined by International Atomic Time (TAI), with leap seconds announced at irregular intervals to compensate for the Earth's slowing rotation. Difference between UTC and GMT Standard Time in.NET. 2021-04-13; 约 1 分钟. Update: Along with many others, I’ve been confused and not completely accurate about the dissimilarity between UTC and GMT. There is a difference. The two are commonly used interchangeably, however, the distinction needs to be clarified for the record. The Difference Explained. GMT is a defined time zone and UTC is a time standard. By convention, the world's weather communities use a twenty four hour clock, similar to 'military' time based on the 0° longitude meridian, also known as the Greenwich meridian. Prior to 1972, this time was called Greenwich Mean Time (GMT) but is now referred to as Coordinated Universal Time or Universal Time Coordinated (UTC).

  • Atomic Times
  • Earth Rotation Times
There are two widely used time standards. One is the rotation of theearth, and the other is the frequency of atomic oscillations (mainly thecesium-133 atom). The earth's rotation is not uniform. Its rate exhibitsboth periodic changes and long term drifts on the order of a second peryear. Atomic standards are the closest approximations we currently haveto a unform time with accuracies on the order of microseconds per year.

Since the advent of atomic time in 1955 there has been a steady transitionfrom reliance on the earth's rotation to the use of atomic time as theprimary standard. Before atomic time, the closest approximation to auniform time was Ephemeris Time (ET), which used the best available theoryof the earth's rotation to remove its known changes in rotation rate. Theuse of Ephemeris Time continued until 1984. It was the time independentvariable for planetary ephemerides until then.

Several important time scales still follow the rotation of the earth, mostnotably civil and sidereal time, but of these are now derived from atomic timethrough a combination of earth rotation theory and actual measurements ofthe earth's rotation and orientation. Chapter 2 of the most recentExplanatory Supplement to the Astronomical Almanac [ref1] gives an extensive summary of time standards and a list of originalliterature references. The kinds of time typically encountered in astronomyare briefly described below.

You will notice that many of the time acronyms are reversed fromtheir full English names. That's because they are acronyms from French (TAI= Temps Atomique International) since France has a long andcontinuing history as a primary source of time standards, now through theBureau International des Poids et Measures.

Atomic Times

TAI - International Atomic Time

International Atomic Time (TAI) is the primary time standard in the worldtoday. It is the combined input of many clocks around the world, eachcorrected for known environmental and relativistic effects. A few clocks,such as the cesium clock ensemble at the U. S. Naval Observatory, carryconsiderable weight in the TAI. In relativistic terms, TAI is anearth-based time since it is defined for a gravitational potential andinertial reference on the surface of the earth. TAI is the standard forthe SI (System International) second. The zero point of TAI wassomewhat arbitrarily defined by early atomic clocks, and its offset fromEphemeris Time was precisely defined as 32.184 seconds for January 1,1977. See TDT below.

UTC - Coordinated Universal Time

Coordinated Universal Time (UTC) is the time broadcast by WWV and otherservices. By definition, UTC and TAI have the same rate, but UTC staysclose to Mean Solar Time by adding integer numbers of seconds, called leapseconds, from time to time. This keeps solar noon at the same UTC(averaged over the year), even though the rotation of the earth is slowingdown. The offset is changed as needed to keep UTC within about 0.9seconds of earth rotation time, UT1. Leap seconds aretypically added once per year at the end of December or June, but they canbe added (or subtracted) at other designated times throughout the year. Theoffset between TAI and UTC is currently 30 seconds, e.g., 20:00:00 UTC =20:00:30 TAI.Before UTC, the time broadcast by WWV and other services was a closeapproximation to Greewich Mean Time (GMT) [ref 2].GMT is an earth rotation time and is now called UT1 or simply UT.

TDT or TT - Terrestrial Dynamic Time

DifferenceBefore atomic clocks, Ephemeris Time (ET) was the closest availableapproximation to a uniform time for planetary motion calculations.Terrestrial Dynamic Time, which is tied to atomic time by a constantoffset of 32.184 seconds, replaced ET at the beginning of 1984. Thepurpose of the offset is to maintain continuity between ET and TDT atthe transition. Planetary motions are now computed using Barycentric Dynamic Time (TDB), which is more uniformthan TT because it accounts for relativistic corrections due to theearth's motion in the gravitational potential of the solar system.There is a subtle relativistic distinction between coordinate time anddynamic time, which is not significant for most practical purposes. Thecounterpart to TT is Geocentric Coordinate Time (TCG) which differs inrate from TT by about 0.7 parts per billion [ref 3].TT and TCG were coincident on January 1, 1977 and now differ by 0.42seconds. The rate difference from TT can be important to long termmeasurements, so make sure you know which time is being used whencomparing observations. Some physical constants are different incoordinate time. You are not likely to encounter TCG in the literature.

TDB - Barycentric Dynamic Time

Barycentric Dynamic Time (TDB) is the same as as Terrestrial Dynamic Time(TT) except for relativistic corrections to move the origin to the solarsystem barycenter. These corrections amount to as much as about 1.6millisends and are periodic with an average of zero. Thedominant terms in this correction are have annual and semi-annualperiods.and JD is the Julian Date. A more accurate formula, with adds termssmaller than 20 microseconds, is given in the Explanatory Supplement tothe Astronomical Almanac [ref 4]. Planetary motionsare now computed using TDB.

There is a subtle relativistic distinction between coordinate time anddynamic time, which is not significant for most practical purposes. Thecounterpart to TDB is Barycentric Coordinate Time (TCB) which differs inrate from TDB by about 15.5 parts per billion [ref 5].TDB and TCB were coincident on January 1, 1977 and now differ by 9.3seconds. The rate difference from TDB can be important to long termmeasurements, so make sure you know which time is being used whencomparing observations. Some physical constants are different incoordinate time. You are not likely to encounter TCB in the literature.

Earth Rotation Times

UT1 - Universal Time

Universal Time (UT1) is a measure of the actual rotation of the earth,independent of observing location. UT1 is essentially the same as the nowdiscontinued Greenwich Mean Time (GMT). It is the observed rotation ofthe earth with respect to the mean sun corrected for the observer'slongitude with respect to the Greenwich Meridian and for the observer'ssmall shift in longitude due to polar motion.

Since the earth's rotation is not uniform, the rate of UT1 is notconstant, and its offset from atomic time is continually changing in a notcompletely predictable way. As of December 1995, UT1 was drifting about0.8 seconds per year with respect to atomic time (TAI or UTC). Since UTCis intentionally incremented by integer seconds (leap seconds) to staywithin 0.9 seconds of UT1, the difference between UT1 and UTC is nevergreater than this. The difference, DUT1 = UT1 - UTC is monitored by theInternational Earth Rotation Service and published weekly in IERS Bulletin A alongwith predictions for a number of months into the future.Note that when a leap second is added to or subtracted from UTC, the valueof DUT1 is discontinuous by one second. UT1 is continuous, and UTC isincremented or decremented by integer seconds to stay witin 0.9 seconds ofUT1.



UT0 (UT-zero) is an observatory-specific version of UT1 in the sense thatUT0 contains the effect of polarmotion on the observed rotation of the earth. Polar motion isequivalent to a change in latitude and longitude of points on the earth'ssurface with respect to the earth's instantaneous rotation axis. SinceUT1 is now determined from observations from an ensemble of obsevatories,often as part of VLBI interferometers, the practical use of UT0 hasdwindled. The conversion from UT1 to a local observatory time withrespect to the mean sun or stars is now done as a set of coordinaterotations that do not explicitly use UT0 as an intermediate step.Difference Between Utc And Gmt

From ref [7]where x and y are the pole offsets published in IERS Bulletin A, andlat and long are the observatory's nominal stationcoordinates. x and y are published in arcseconds sothey must be converted to time (one time second = 15 arcseconds).


Difference Between Utc And GmtBetweenUT2 appears to be of mostly historical interest. Before 1972 the timebroadcast services kept their time signals within 0.1 seconds of UT2,which is UT1 with annual and semiannual variations in the earth'srotation removed. The formal relation between UT1 and UT2 iswhereis the Besselian day fraction, and MJD is the Modified Julian Date (JulianDate - 2400000.5). See ref [6] and the Explanatory Supplementto IERS Bulletins A and B.

GMST - Greenwich Mean Sidereal Time

Sidereal time is the measure of the earth's rotation with respect todistant celestial objects. Compare this to UT1, which is the rotation ofthe earth with respect to the mean position of the sun. One siderealsecond is approximately 365.25/366.25 of a UT1 second. In other words,there is one more day in a sidereal year than in a solar year.

By convention, the reference points for Greenwich Sidereal Timeare the Greenwich Meridian and the vernal equinox (the intersectionof the planes of the earth's equator and the earth's orbit, the ecliptic).The Greenwich sidereal day begins when the vernal equinox is on theGreenwich Meridian. Greenwich Mean Sidereal Time (GMST) is the hour angleof the average position of the vernal equinox, neglecting short termmotions of the equinox due to nutation.

In conformance with IAU conventions for the motion of the earth's equatorand equinox [ref 7] GMST is linked directly to UT1through the equationwhere T is in Julian centuries from 2000 Jan. 1 12h UT1It might seem strange that UT1, a solar time, isdetermined by measuring the earth's rotation with respect to distantcelestial objects, and GMST, a sidereal time, is derived from it. Thisoddity is mainly due our choice of solar time in defining the atomic timesecond. Hence, small variations of the earth's rotation are more easilypublished as (UT1 - Atomic Time) differences. In practice, of course,some form of sidereal time is involved in measuring UT1.

GAST - Greenwich Apparent Sidereal Time

Greenwich Apparent Sidereal Time (GAST) is Greenwich Mean Sidereal Time(GMST) corrected for the shift in the position of thevernal equinox due to nutation. Nutationis the mathematically predictable change in the direction of the earth'saxis of rotation due to changing external torques from the sun, moon andplanets. The smoothly varying part of the change in the earth'sorientation (precession) is alreadyaccounted for in GMST. The right ascension component of nutation iscalled the 'equation of the equinoxes' [ref 9].

LMST - Local Mean Sidereal Time

Local Mean Sidereal time is GMST plus the observer's longitudemeasured positive to the east of Greenwich. This is the time commonlydisplayed on an observatory's sidereal clock.Should i use utc or gmt

LST - Local Sidereal Time

The definition of Local Sidereal Time given in the glossary of theExplanatory Supplement to the Astronomical Almanac is 'the local hourangle of a catalog equinox.' This fits the common text book definitionwhere the right ascension can be specified in one of the catalogcoordinate systems B1950 (FK4) or J2000 (FK5), for example. In practice,LST is used more loosely to mean either LMST or 'Local Apparent SiderealTime' = GAST + (observer's east longitude). The operational definitionprobably varies from one observatory to the next.

IAU Source Code

You can find FORTRAN source code for computing TAI - UTC, TDB - TT, GMSTfor a given UT1 and the Equation of the equinoxes at thisIAUweb site.

Utc To Gmt +8


Utc To Gmt

Last updated January, 30, 1996.