UTC(NIST) is the coordinated universal time scale maintained at NIST. The UTC(NIST) time scale comprises an ensemble of cesium beam and hydrogen maser atomic clocks, which are regularly calibrated by the NIST primary frequency standard. The number of clocks in the time scale varies, but is typically around ten. The outputs of the clocks are combined into a single signal by using a weighted average. The most stable clocks are assigned the most weight. The clocks in the UTC(NIST) time scale also contribute to the International Atomic Time (TAI) and Coordinated Universal Time (UTC).
UTC(NIST) serves as a national standard for frequency, time interval, and time-of-day. It is distributed through the NIST time and frequency services and continuously compared to the time and frequency standards located around the world.
International Atomic Time (TAI) is an international time scale that is computed by taking the weighted average of more than 300 atomic clocks. These clocks are located at more than 60 timing laboratories around the world. The most stable clocks receive the most weight in the calculation, but the maximum weight assigned to any clock is limited to A / N, where A = 2.5 and N is the number of clocks. Thus, if N = 300, the maximum weight is 0.83 %. While the stability of TAI is achieved by this weighted average, the accuracy of TAI is derived from data from primary frequency standards, which are clocks built at several national metrology institutes. TAI is computed by the International Bureau of Weights and Measures (BIPM) located near Paris, France.
Coordinated Universal Time (UTC) is based on TAI, but it is adjusted by leap seconds to account for the difference between the definition of the second and the rotation of Earth. This correction keeps UTC in conjunction with the apparent position of the Sun and the stars, and it is the standard used for all general timekeeping applications. We publish the current difference between UTC and TAI in our Time Scale Data and Bulletin Archive.
For more information, see the section on leap seconds.
No clock keeps the "official" version of UTC, because TAI and UTC are "paper" time scales that can only be calculated after all of the data from the international contributors are received. However, NIST and other laboratories maintain real-time versions of UTC that are often within just a few nanoseconds of the UTC calculations. The international contributors to TAI, and their offsets, are published in the BIPM Circular-T, which is a document updated monthly.
The time-of-day expressed by UTC is the time at the prime meridian (0° longitude) located near Greenwich, England. The time in local time zones can be expressed as an offset from UTC. For instance, in the United States, eastern standard time (EST) is five hours behind UTC and can be expressed as UTC - 5. Coordinated Universal Time is not adjusted during daylight saving time, so eastern daylight time (EDT) is only 4 hours behind UTC, or UTC - 4.
In 1970, the Coordinated Universal Time system was devised by an international advisory group of technical experts within the International Telecommunication Union (ITU). The ITU felt it was best to designate a single abbreviation for use in all languages in order to minimize confusion. For example, in English the abbreviation for coordinated universal time would be CUT, while in French the abbreviation for "temps universel coordonné" would be TUC. To avoid appearing to favor any particular language, the abbreviation UTC was selected.
Greenwich Mean Time (GMT) originally referred to the mean solar time at the Royal Observatory in Greenwich, England. As an astronomical time scale, it followed the irregular motion of Earth. The modern term for this astronomical time is UT1. The term GMT is now more commonly used to refer to the time zone at the prime meridian (0° longitude), in which case it is being used as a local representation of Coordinated Universal Time (UTC) and not UT1. However, UTC is adjusted with leap seconds to always be within less than one second of UT1, so either use of GMT can be considered equivalent to Coordinated Universal Time (UTC) when fractions of a second are not important.
The United States Naval Observatory (USNO) maintains the U. S. Department of Defense reference for time and time interval. USNO has an ensemble of atomic clocks, which is used to derive a time scale called UTC(USNO). The clocks in the ensemble contribute to International Atomic Time (TAI) and Coordinated Universal Time (UTC). UTC(USNO) and UTC(NIST) are kept in very close agreement, typically to within 20 nanoseconds, and both can be considered official sources for time in the United States. Recent differences between the two time scales are published in the NIST Time Scale Data Archive.
The Global Positioning System (GPS) is a constellation of satellites each carrying multiple atomic clocks. The time on each satellite is derived by steering the on-board atomic clocks to the time scale at the GPS Master Control Station, which is monitored and compared to UTC(USNO). Since GPS time does not adjust for leap seconds, it is ahead of UTC(USNO) by the integer number of leap seconds that have occurred since January 6, 1980 plus or minus a small number of nanoseconds. However, the time offset from UTC is contained in the GPS broadcast message and is usually applied automatically by GPS receivers.
Beginning in the 1920s, nautical time zones were referred to with alphabetic characters. The Z zone is equivalent to Coordinated Universal time (UTC), which is the time at the prime or "zero" meridian (0° longitude). The phonetic alphabet refers to Z as "Zulu". It is still used some settings (i.e. military, nautical and aviation) to refer to UTC.