Coordinated Universal Time (UTC)
How time laboratories, clock data and the BIPM work together to produce and maintain the international reference time scale.
Why it matters
One time scale connecting systems around the world
UTC underpins much of modern life.
It enables navigation systems, telecommunications networks, financial transactions, scientific facilities and digital infrastructure to agree precisely on time. Most people never encounter UTC directly, but rely on it whenever systems in different places need to stay synchronized.
Navigation · Telecommunications · Digital infrastructure · Science · Financial systems · Civil time
Building UTC
From local time scales to UTC
How participating laboratories and the BIPM work together to produce and maintain the international reference time scale.
Local real-time realizations
Participating laboratories maintain local real-time realizations of UTC, known as UTC(k), and compare them internationally using calibrated time-transfer techniques.
International Atomic Time (TAI)
Using contributed clock and time-transfer data, the BIPM computes TAI, a continuous atomic time scale based on realizations of the SI second.
From TAI to UTC
UTC is then derived from TAI by applying leap seconds to maintain agreement with time based on the Earth’s rotation.
UTC results
Each month, the BIPM publishes the UTC results and the information laboratories need to compare UTC(k) with UTC and establish traceability to the international reference time scale.
UTC data and documentation
Access UTC results, technical information and authoritative resources on the SI unit of time.
Circular T
Access the monthly publication containing the differences between UTC and the local real-time realizations maintained by participating laboratories.
Access Circular T →The SI second
Explore the definition of the SI unit of time, how UTC provides access to a high-accuracy realization of the SI second, and information on how the second is realized in practice.
Explore the SI second →Frequently asked questions
Find quick answers to common questions about UTC and international timekeeping.
Why do we need UTC?
UTC is the international reference time scale, enabling navigation systems, telecommunications networks, digital infrastructure, scientific facilities and civil time services to remain synchronized.
What is UTC(k)?
UTC(k) is a local real-time realization of UTC maintained by a participating National Metrology Institute or designated time laboratory, where k identifies the institute. These laboratories contribute clock and time-transfer data to the BIPM, which publishes the differences between UTC and UTC(k) in Circular T. This allows each laboratory to compare its local time scale with the international reference time scale and establish traceability to UTC.
What is TAI, and how does it relate to UTC?
International Atomic Time (TAI) is a continuous atomic time scale computed by the BIPM from clock and time-transfer data contributed by participating laboratories. It is an intermediate step in the computation of UTC. By adding leap seconds to TAI, UTC is kept in agreement with time based on the rotation of the Earth. TAI is also a realization of Terrestrial Time (TT), with the same rate as TT.
Why are leap seconds used in UTC?
Leap seconds are used to maintain the relationship between UTC and time based on the rotation of the Earth.
How often are UTC results published?
The results of the UTC computation are published monthly by the BIPM in Circular T.
How does Rapid UTC relate to UTC?
Rapid UTC, or UTCr, is a weekly rapid solution that complements UTC by providing participating laboratories with earlier operational information between monthly Circular T publications.
Read the detailed FAQs
Access further information about UTC, leap seconds and the future development of international timekeeping.
Continue exploring international timekeeping
Explore the BIPM Time Department
Learn more about Rapid UTC, TT(BIPM), future developments in timekeeping and the wider work of the BIPM Time Department.
Return to the Time Department page →