Basically, the Universal Time systems are coordinated world-wide standards, whereas GMT was a national standard that was used as a de facto world standard.

Greenwich Mean Time is the mean solar time plus 12 hours as observed from Greenwich, England. (

*Mean solar* means that you use average out the seasonal changes due to the earth's elliptical orbit by using a theoretical

*mean sun* rather strictly observing the real one. Otherwise day length would vary by over 1% throughout the year.)

But by 1972, atomic clocks had spread around the world and were used as centralized time-keeping fixtures. Around this time, a second was also redefined in terms of electron transitions in a cesium atom, rather than a certain fraction of a mean solar day.

This lead to the establishment of a coordinated

*atomic clock time* called Tempes Atomique International (TAI).

But the earth doesn't always spin at the same rate... this is due to a combination of factors including polar eccentricity, periodic seasonal variations due to the ellipical orbit around the sun, the slowing down of the earth (that is, momentum transfer to the earth-moon system), plus other unexplained empirically observed changes.

So this "pure" theoretical time (TAI) would drift farther and farther out of alignment with the real world.

So a new standard called Coordinated Universal Time (UTC) was created. It is TAI with an offset of a certain number of

*leap seconds,* currently 22 (though there was a 10 second difference between TAI and UTC due to a variety of slippages). These leap seconds adjust the atomic-clock time back into the actual observed mean solar time with respect to the prime meridian.

For what it's worth, UTC is in fact just one of several of the Universal Time family of time standards. And if you want to be more accurate than tenths of seconds, you should also specifiy

**which** UT

*i* you are using... UT0, UT1, UT2, and so on. And since UTC is actually determined from each individual atomic clock, to be perfectly pedantic you should also specify

**which** atomic clock you are using, such as UTC(

USNO).

The concept of a virtual time scale can be difficult to grasp, but perhaps becomes easier when you consider that none of the clocks in the UTC ensemble reside at the BIPM and thus the BIPM doesn’t measure any physical clock signals. Instead, the BIPM calculates UTC from measurement data that is sent to them by the more than 80 laboratories mentioned earlier. Several different methods are available for measuring and collecting clock data to send to the BIPM. For example, NIST submits its data to the BIPM using the

satellite technique described here.

Once a month, after all clock measurements have been collected and after all calculations are complete, the BIPM publishes the results in an on-line document known as

*Circular T*. This document lists the time difference between UTC and each of the local time scales that are subsets of UTC, such as UTC(NIST). In

*Circular T* parlance, the local time scales are known as UTC(

*k*), where k is a variable that designates the name of the local time scale’s laboratory, and where values for UTC – UTC(

*k*), are reported at 5-day intervals. The publication of these values is extremely important for ensuring that UTC is indeed “coordinated” around the world, because it verifies the accuracy of each UTC(

*k*) time scale with respect to UTC. By doing so it allows each local UTC(k) time scale to establish metrological traceability to both UTC and to the SI second.

To make data available more frequently than once per month, the BIPM also publishes an unofficial time scale called Rapid UTC, or UTCr. Rapid UTC is published once per week, with values of UTCr – UTC(

*k*) reported at 1-day intervals. Currently (November 2020), NIST and more than 60 other laboratories participate in UTCr.

Although the values computed by UTC and UTCr are not known until after the fact, UTC(NIST) is continuously adjusted using forward prediction techniques that anticipate where UTC will be when the

*Circular T* is published. As a result, UTC(NIST) is an excellent approximation of UTC that has the advantage of producing physical signals in real time, and that typically differs from UTC by less than three billionths of a second (3 ns).

Of course, in practice, only a very, very few people have any real reason to be that OCD. The rest of us can muddle along saying "quarter to four."

See:

https://cesium.clock.org/~jss/work/time.html
See:

https://www.nist.gov/pml/time-and-frequency-division/time-realization/utcnist-time-scale-0/introduction-utcnist
Universal Time to Local Time Conversion Chart:

See:

http://www.timebie.com/std/utc.php
You're welcome to read my offering concerning:

Is not UTC time standard based on cesium 133 atoms and not rotation of the earth? from August 15, 2021 in LiveScience.

Hartmann352