History of UTC: How the World Agreed on Time

The Washington Meridian Conference, 1884

Twenty-five nations met in Washington, D.C., in October 1884 to solve a problem railways and telegraphs had made urgent: every town set its own noon by the sun, so a timetable from one city meant nothing in the next. They voted to place the prime meridian at the Royal Observatory in Greenwich, zero degrees longitude. The vote was 22-1. San Domingo voted against; France and Brazil abstained.

Greenwich Mean Time became the world’s reference. Ships set their chronometers to it. Railway timetables adopted it. But GMT was still solar time, defined by the rotation of the Earth. And the Earth does not rotate steadily.

1955: The first caesium atomic clock

By the 1930s astronomers knew the Earth’s rotation was slowing. GMT was drifting. Quartz clocks were already more stable than the planet.

In 1955 Louis Essen and his team at the UK National Physical Laboratory built the first caesium atomic clock. It kept time by counting the transition frequency of caesium-133 atoms. The atomic second was defined as 9,192,631,770 transitions. That number was chosen to match the length of the ephemeris second, the astronomical second based on Earth’s orbit. It kept the new atomic time aligned with the old astronomical time, at least at first.

1958: International Atomic Time (TAI) established

National laboratories in the US, UK, Switzerland, and France began running atomic clocks. The Bureau International de l’Heure in Paris combined their readings into a single timescale: International Atomic Time, TAI.

TAI was continuous. It did not slow down. It did not speed up. It ticked at exactly 9,192,631,770 caesium transitions per second, forever.

The Earth kept slowing. By 1958 astronomical time (UT1) and TAI were already a few tenths of a second apart. The gap was growing.

1960: UTC is born

The International Radio Consultative Committee adopted Coordinated Universal Time in 1960. The idea: keep the atomic precision of TAI but adjust it in small steps to stay within 0.1 seconds of astronomical time.

The first version of UTC used rate adjustments. The frequency of the time signal was shifted slightly to match Earth’s rotation. Broadcasters and navigators needed a clean signal, not a drifting one. In 1972 the system changed.

1972: The leap-second system begins

Instead of adjusting the rate, the CCIR introduced the leap second. UTC would now run at the same rate as TAI, the full atomic second, but whenever Earth’s rotation fell too far behind a whole second would be inserted.

The first leap second was added on 30 June 1972 at 23:59:60 UTC. Since then 27 leap seconds have been added, all positive. None have been removed. The most recent was on 31 December 2016.

The rule: UTC stays within 0.9 seconds of UT1. The International Earth Rotation and Reference Systems Service decides when a leap second is needed and announces it roughly six months ahead in Bulletin C. Leap seconds are inserted at the end of June or December. The sequence runs 23:59:59, 23:59:60, 00:00:00. The extra second exists only in UTC.

The name debate: why ‘UTC’ and not ‘CUT’ or ‘TUC’

The French delegation wanted TUC (Temps Universel Coordonné). English-speaking countries wanted CUT (Coordinated Universal Time). Neither side would yield.

The compromise: UTC. It is not an acronym for either language’s word order. It is a neutral abbreviation that works in both. The CCIR adopted it in 1967 and it stuck.

2015-2022: The movement to abolish leap seconds

Leap seconds cause problems. NTP servers must handle the extra second. Some systems smear it over hours. Others repeat the last second. The 2012 leap second caused outages at Reddit, LinkedIn, and Qantas.

The International Telecommunication Union considered abolition in 2015 but deferred the decision. In 2022 the General Conference on Weights and Measures passed Resolution 4: leap seconds will be discontinued by 2035 at the latest. The plan allows the difference between UTC and UT1 to grow beyond one minute. The exact replacement mechanism is still under discussion. Russia opposed the change because its GLONASS navigation system relies on the current leap-second structure. Check with the ITU for the latest status.

UTC today: the world’s reference timescale

The Bureau International des Poids et Mesures in Paris computes UTC from about 450 atomic clocks in 80 national laboratories worldwide. The clocks are compared using GPS and satellite links. The BIPM publishes the difference between each laboratory’s local time and UTC every month in Circular T.

UTC is not a time zone. It is a timescale. The IANA time zone database includes Etc/UTC as a zone identifier for systems that need one. UTC never observes daylight saving time. Its offset is always UTC+0.

The table below shows how UTC relates to major cities on 15 January 2026 and 15 July 2026:

Only cities in countries that observe daylight saving time change their offset between January and July. UTC itself does not change.

How atomic clocks define the second: 9,192,631,770 transitions

The second is defined by the International System of Units as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the caesium-133 atom. That number was chosen in 1967 to match the length of the ephemeris second as measured in 1957-1958. It has not changed since.

A caesium fountain clock cools caesium atoms to near absolute zero, launches them upward, and measures the microwave frequency that causes them to change state.

Where atomic clocks live: national laboratories worldwide

The major contributors to UTC include:

• National Institute of Standards and Technology (NIST) in Boulder, Colorado
• Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig, Germany
• National Physical Laboratory (NPL) in Teddington, UK
• Observatoire de Paris (SYRTE) in Paris, France
• National Institute of Information and Communications Technology (NICT) in Tokyo, Japan

Each laboratory runs multiple caesium fountain clocks and hydrogen masers. Their results are combined to produce TAI, which is then adjusted by leap seconds to produce UTC.

When did UTC replace GMT?

1960 for the name, 1972 for the leap-second system that made UTC what it is today.

GMT is still used as a time zone name. The UK uses GMT (UTC+0) in winter and BST (UTC+1) in summer. The BBC still says “GMT” on its World Service but means UTC. For most practical purposes, booking a meeting, logging a server event, filing a flight plan, UTC and GMT are interchangeable. The difference matters only when you need the precision of atomic time or when you are dealing with leap seconds.

Aviation calls it Zulu time. The military calls it Zulu time. Computing calls it Unix time (seconds since 1970-01-01T00:00:00Z, ignoring leap seconds). They all point to the same reference.