Coordinated Universal Time

Coordinated Universal Time
Name	Coordinated Universal Time
Abbreviation	UTC
Introduced	1960s
Predecessor	Greenwich Mean Time
Basis	Atomic time and astronomical observations
Authority	International Bureau of Weights and Measures; International Telecommunication Union

Contents

Coordinated Universal Time. Coordinated Universal Time is the principal international civil time standard that synchronizes clocks worldwide for navigation, telecommunications, finance and scientific research. It unites atomic timekeeping from institutions like the International Bureau of Weights and Measures with astronomical references tied to the Royal Observatory, Greenwich and observatories such as the U.S. Naval Observatory, enabling consistent time stamps for systems operated by entities including NASA, European Space Agency, International Telecommunication Union and International Civil Aviation Organization.

Definition and purpose

The evolution of Coordinated Universal Time traces through milestones involving Sir George Biddell Airy, the establishment of the Prime Meridian conference at International Meridian Conference, adoption of Greenwich Mean Time by shipping companies like Cunard Line and navies including the Royal Navy and the United States Navy. Advances in physics at institutions such as National Physical Laboratory (UK), Bureau International de l'Heure, International Bureau of Weights and Measures and researchers like Isidor Rabi and Louis Essen led to the atomic definition of the second adopted by the International Committee for Weights and Measures and later by the General Conference on Weights and Measures. The establishment of UTC in the 1960s involved coordination among International Telecommunication Union, International Astronomical Union, International Geophysical Year participants, and observatories including the Paris Observatory and the U.S. Naval Observatory, culminating in protocols used by Global Positioning System, the GLONASS program, the BeiDou Navigation Satellite System, and the Galileo (satellite navigation) system.

UTC is realized through an ensemble of atomic clocks operated by national metrology institutes such as National Institute of Standards and Technology, Physikalisch-Technische Bundesanstalt, Laboratoire national de métrologie et d'essais, National Metrology Institute of Japan, and coordinated by the International Bureau of Weights and Measures which publishes International Atomic Time and coordinates with the International Earth Rotation and Reference Systems Service for Earth orientation parameters. The SI second is defined by the hyperfine transition of cesium-133 atoms, a measurement refined by laboratories like National Physical Laboratory (UK) and technologies from firms such as Agilent Technologies and Keysight Technologies. Time dissemination standards include protocols from IETF like Network Time Protocol and Precision Time Protocol, and telecommunication standards from ITU-R and ITU-T.

To keep UTC close to mean solar time, leap seconds are inserted or, theoretically, removed based on measurements from the International Earth Rotation and Reference Systems Service and decisions by the International Telecommunication Union Radiocommunication Sector. Leap seconds were introduced starting in 1972 and coordinated with agencies including the U.S. Naval Observatory and observatories such as USNO and Paris Observatory. Proposals to abolish leap seconds have been debated at assemblies like ITU Plenipotentiary Conference and General Conference on Weights and Measures, with stakeholders including Google, Facebook, Amazon Web Services, European Broadcasting Union, International Air Transport Association and scientific bodies such as CERN and European Southern Observatory weighing operational impacts.

Civil time zones worldwide are defined as offsets from UTC by governments of countries such as the United Kingdom, United States, China, Russia, India, Brazil, Australia, Canada, Japan, Germany, and territories like Greenwich Mean Time zone jurisdictions, with regional organizations like the European Union coordinating cross-border schedules. Aviation and maritime practices under International Civil Aviation Organization and International Maritime Organization use UTC for flight plans and navigation, while financial institutions including Federal Reserve System, European Central Bank, Bank of Japan and exchanges like NASDAQ reference UTC for settlement windows. Timekeeping in networks follows UTC offsets like UTC+1 (Central European Time), UTC−5 (Eastern Standard Time) and other standards managed by organizations like IANA which maintains the tz database used by operating systems such as Linux, Windows, macOS, and distributions from Red Hat and Canonical.

UTC is disseminated via radio services like WWV, DCF77, MSF (time signal), RBU, satellite systems including GPS and Galileo, and internet protocols such as Network Time Protocol and Precision Time Protocol. Telecommunication carriers including AT&T, Verizon Communications, Deutsche Telekom, Orange S.A., and China Telecom rely on UTC synchronization for packet networks and services; broadcasters like BBC and NHK schedule programs to UTC-referenced schedules. Critical infrastructure operators in sectors such as SWIFT, Bloomberg L.P., NASDAQ OMX Group, and grid operators coordinate with metrology institutes to maintain traceability to UTC.

Legal definitions and statutory references to UTC appear in legislation and regulations of countries including the United Kingdom, United States, France, Germany, Japan, Australia, Canada, and supranational frameworks like the European Union directives; international agreements and standards bodies such as the International Telecommunication Union, International Bureau of Weights and Measures, International Organization for Standardization, and the International Electrotechnical Commission provide the normative framework for UTC. Disputes over civil adoption or changes, such as abolition of leap seconds, involve national regulators like the Federal Communications Commission and intergovernmental bodies including the United Nations.