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Coordinated Universal Time (UTC) Explained

Coordinated Universal Time (UTC) is the basis for civil time in many places worldwide. Many devices for measuring and showing time use this 24-hour time scale, which is determined using highly precise atomic clocks. Time zones around the world are expressed as positive or negative offsets from UTC. The hours, minutes, and seconds that UTC expresses is kept close to the mean solar time at the earth's prime meridian (zero degrees longitude) located near Greenwich, England.

UTC is often casually interchanged with Greenwich Mean Time (GMT) and occasionally UT1 (Universal Time’s principal form) when referred to without counting precise accuracies. However, it is important to know that there are differences between these terms, particularly when considering fractions of a second.

About UTC

UTC is the time broadcast across the world since 1972. It is popularly referred to as GMT and at times UT1. UTC is also the time system used in aviation and is informally known as Zulu Time to avoid confusion about time zones and daylight saving time. The world's timing centers agreed to keep their real-time timescales closely synchronized (“coordinated”) with UTC. Hence, all these atomic timescales are called UTC. UTC relates to solar motion.  A constant day of exactly 24 hours is used for civil time keeping purposes.

The International Bureau of Weights and Measures makes use of data from the time laboratories to provide the international standard UTC, which is accurate to about a nanosecond (billionth of a second) per day. A UTC second’s length is defined in terms of an atomic transition of the element cesium under specific conditions and is not directly related to any astronomical phenomena.

Radio stations that broadcast time, such as the National Institute of Standards and Technology’s (NIST) radio stations WWV and WWVH, make use of UTC. It can also be obtained from the Global Positioning System (GPS) satellites. The difference between UTC and UT1 is made available electronically and broadcast so that navigators can obtain UT1.

UTC is the basis for civil standard time in many countries, including the United States and its territories. Standard time within US time zones is an integral number of hours offset from UTC. UTC is equivalent to the civil time for countries such as (but not exclusive to):

UTC is also casually referred to as GMT, which is the civil time scale for places such as the United Kingdom and the Republic of Ireland during the non-daylight saving period.


UTC is commonly referred to as Greenwich Mean Time (GMT) when not counting the precise accuracy regarding fractions of a second. GMT is a historical term that has been widely used in many ways. GMT was first adopted as the world’s time standard at the Washington Meridian Conference in 1884. GMT is no longer the basis for civil time but is now loosely interchanged with UTC to refer to time kept on the Greenwich meridian (longitude zero). Places such as the United Kingdom observe GMT during the non-daylight saving period.

The use of GMT is no longer recommended in technical contexts because of its ambiguity. A day of GMT began at noon (12:00) in astronomical and nautical almanacs prior to 1925 (the day of UTC starts at midnight (00:00), unlike GMT). This reckoning of GMT is now called Greenwich Mean Astronomical Time and is no longer used. People using old editions of the almanacs for historical research should be aware of the previous convention.

UT1 and UTC

According to the US Naval Observatory (USNO), one can think of UT1 as a time that the earth’s rotation determines. One has no control over it, whereas UTC is a human invention. It is relatively easy to manufacture highly precise clocks that keep UTC, while the only “clock” keeping UT1 precisely is the earth itself.

Nevertheless, it is desirable that the civil time scale should not be very different from the earth's time. UTC cannot differ from UT1 by more than 0.9 seconds. A one-second change called a “leap second” is introduced into UTC if it appears that the difference between the two kinds of time may approach this limit. This occurs on average about once every year to a year and a half.

UTC and Leap Seconds

UTC is kept within 0.9 seconds of UT1 by leap seconds, either positive (23:59:60 added before 00:00:00) or negative (23:59:59 omitted; no actual case has yet occurred and probably will occur), at the end of June or December. UTC runs at the same frequency as International Atomic Time (TAI), a statistical time scale based on many atomic clocks. However, it differs from TAI by an integral number of seconds. This difference increases when leap seconds occur.

The International Earth Rotation and Reference System Service (IERS) observes the earth's rotation and sends a bulletin message six months in advance (January or July).  The message reports whether or not to add a leap second in the end of June and December. IERS schedules a leap second as needed to keep the time difference between atomic clocks and Earth’s rotation to below 0.9 seconds.

Note: timeanddate.com wishes to acknowledge that some of the information in this article is courtesy of the National Institute of Standards and Technology (NIST) and the United States Naval Observatory (USNO).


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