A pilot over the Atlantic filed a flight plan in Zulu time. A trader in Singapore executed a forex order. A server in São Paulo wrote a log entry. All three happened at the same moment, and all three used the same reference: UTC. That reference does not come from one master clock. It comes from a distributed system of atomic clocks, scientific committees, and database maintainers who rarely meet in the same room.
Five organisations share the work. Each has a specific job, and each publishes a document or dataset the rest of the world relies on.
BIPM: Computing UTC from 450 atomic clocks
The Bureau International des Poids et Mesures (BIPM), based near Paris, does the maths. Every month, the BIPM collects time readings from roughly 450 atomic clocks spread across 80 national laboratories. Caesium fountain clocks, hydrogen masers, and rubidium clocks all contribute.
The BIPM does not own any of these clocks. It receives data, applies corrections for gravitational redshift and clock drift, and computes a weighted average. That average is International Atomic Time (TAI), the most stable timescale ever built. TAI ticks at exactly 9,192,631,770 cycles of a caesium atom per moment and never stops.
UTC is then derived from TAI by subtracting an integer number of moments. As of January 2026, that number is 37. This offset exists because Earth’s rotation is slowing down, and UTC must stay within 0.9 moments of astronomical time (UT1). The extra moments are the leap seconds.
The BIPM publishes its results monthly in a document called Circular T. Circular T lists the difference between UTC and each contributing clock, the computed UTC, TAI offset, and the steering corrections applied. Any laboratory that operates a time service uses Circular T to calibrate its own UTC realisation.
IERS: Deciding when leap seconds happen
The International Earth Rotation and Reference Systems Service (IERS) monitors Earth’s rotation. It measures UT1, the actual angle Earth has turned, using Very Long Baseline Interferometry (VLBI), laser ranging to the Moon, and GPS satellite orbits.
When UT1 drifts more than 0.6 moments away from UTC, the IERS decides a leap second is needed. It announces the decision in Bulletin C, published in January and July each year. If no leap second is needed, Bulletin C says so. If one is needed, it gives the exact date, always 30 June or 31 December, six months in advance.
The IERS has never announced a negative leap second. All 27 leap seconds since 1972 have been positive, meaning a moment was added. The most recent was at the end of 2016.
ITU: The rules for broadcasting UTC
The International Telecommunication Union (ITU), a United Nations agency, writes the rules for broadcasting time signals. Recommendation ITU-R TF.460 specifies how UTC should be disseminated via radio, satellite, and network protocols. It defines the format of time codes, the accuracy requirements, and the relationship between UTC and other timescales.
When a national laboratory transmits a time signal, for example, the US Naval Observatory’s WWVB or France’s TDF, it follows ITU-R TF.460. The ITU also convenes the World Radiocommunication Conferences where member states vote on changes to UTC. In 2022, the General Conference on Weights and Measures (CGPM) passed Resolution 4 to plan for the abolition of leap seconds by 2035. The ITU will implement the technical side of that decision.
IANA: The time zone database
The Internet Assigned Numbers Authority (IANA) maintains the tz database (also called tzdata or zoneinfo). This is the dataset that tells operating systems, programming languages, and cloud platforms what the local time is anywhere on Earth, at any historical date.
IANA does not set UTC. But it provides the identifier Etc/UTC for the UTC timescale itself. When a developer stores a timestamp in a database or sets a server’s time zone to Etc/UTC, they are using IANA’s definition.
The tz database also contains the rules for daylight saving time, historical time zone changes, and the offset from UTC for every inhabited location. It is updated several times a year as governments change their time zone laws. Without IANA, a phone would not know whether London is on UTC+0 or UTC+1 in July.
National laboratories: The clock operators
The atomic clocks that feed the BIPM are owned and operated by national laboratories. Examples include the US Naval Observatory (USNO), the National Institute of Standards and Technology (NIST) in the US, the Physikalisch-Technische Bundesanstalt (PTB) in Germany, and the National Metrology Institute of Japan (NMIJ).
Each institute maintains its own realisation of UTC, often called UTC(k) where k is the lab’s code, for instance, UTC(USNO) or UTC(PTB). These realisations are the clocks that actually tick. The BIPM’s computed UTC is an ideal paper clock; the institutes provide the physical signals that computers, GPS satellites, and broadcast transmitters use.
How the UTC system functions
The system is circular.
- National laboratories operate atomic clocks and send data to the BIPM.
- The BIPM computes TAI and UTC, publishes Circular T.
- The IERS monitors Earth’s rotation, publishes Bulletin C when a leap second is needed.
- The ITU sets the broadcast standard (ITU-R TF.460) that laboratories follow when transmitting time.
- IANA packages the resulting rules into the tz database that computers read.
No one body can change UTC alone. A leap second requires the IERS to detect the drift, the BIPM to incorporate it into the calculation, and the ITU to authorise the broadcast format. The tz database then records the new offset for applications that need to convert UTC to local time.
Circular T and Bulletin C: The key publications
To check the state of UTC at any moment, two documents matter.
Circular T is published monthly by the BIPM. It contains the computed difference between UTC and TAI (37 moments as of January 2026), the performance of each contributing clock, and the steering corrections applied to keep UTC aligned. It is the authoritative source for the exact value of UTC.
Bulletin C is published twice a year by the IERS in January and July. It announces whether a leap second will occur at the end of June or December. Bulletin C is the document that tells the world when to add or remove a moment.
Both are freely available online.
How UTC is calculated: Atomic clocks and the BIPM
The BIPM’s calculation is a free-running atomic time scale. Each national laboratory sends its clock readings to the BIPM, which processes them using an algorithm that weights each clock by its stability. More stable clocks get more influence. The resulting TAI ticks at a rate that is more uniform than any single clock.
UTC is then TAI minus an integer number of moments. The integer changes only when a leap second is inserted. Between leap seconds, UTC and TAI advance at exactly the same rate. The current difference is 37 moments, and it will remain 37 until the next leap second.
Why a single time reference matters
Without UTC, every computer network, airline schedule, stock exchange, and scientific experiment would have to negotiate its own time base. That is exactly what happened before 1960, when every country used its own local mean time and railways ran on dozens of different clocks.
UTC solves that by providing a single, stable, universally agreed tick. It does not matter whether local time is UTC+8 (Singapore) or UTC−5 (New York in winter). Knowing the offset from UTC lets any time convert to any other. The organisations described here ensure that offset is accurate to within nanoseconds.