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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):

• Iceland.


• Ghana.


• Liberia.


• Mali.


• Mauritania.


• Morocco.


• Senegal.

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.

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