UTC (Coordinated Universal Time)
utc
The primary time standard by which the world regulates clocks and time, serving as the basis for civil timekeeping globally.
TAI (International Atomic Time)
tai
Definition
TAI (Temps Atomique International, or International Atomic Time) is a timescale produced by statistically combining data from approximately 450 atomic clocks maintained by over 80 national metrology laboratories. The International Bureau of Weights and Measures (BIPM) in Paris performs the computation. TAI is a continuous timescale with no leap seconds, running from its epoch of 1 January 1958 00:00:00 (UT2).
Relationship to UTC
UTC is derived from TAI by applying leap-second adjustments. As of 2026, UTC = TAI - 37 seconds. This offset is the sum of 27 leap seconds inserted since 1972 and the initial 10-second difference established when UTC was introduced. After the planned abolition of leap seconds in 2035, the 37-second gap will be frozen, effectively making UTC a fixed-offset derivative of TAI.
How It Is Computed
TAI is calculated retrospectively. Data from participating clocks are collected over roughly one month, then processed statistically and published in a monthly bulletin called Circular T. No real-time TAI signal exists; instead, each laboratory maintains its own real-time approximation called UTC(k). In Japan, NICT maintains UTC(NICT) as the local realization of the standard.
Was this article helpful?
Related Terms
Atomic Clock
atomic clock
An atomic clock measures time by using the quantum-mechanical transition frequency of atoms as its reference, achieving accuracies on the order of one second in hundreds of millions of years and forming the backbone of modern timekeeping.
Leap Second
leap-second
A one-second adjustment applied to UTC to keep it aligned with the Earth's irregular rotation.
Related Articles
How Standard Time Is Determined - The International System Behind UTC
UTC is not produced by a single clock but by 80+ research institutions worldwide whose data is averaged by the BIPM in Paris. This article explains TAI, leap second decisions, the distribution channels from satellites to NTP, and the upcoming redefinition of the second.
The International Date Line - Why the Same Moment Has Different Dates
The International Date Line zigzags through the Pacific along roughly 180 degrees longitude, separating today from tomorrow. This article explains why the line is not straight, what happens when ships and planes cross it, and why the world's earliest sunrise lands in Kiribati.
The History of Standard Time - How Railways Forced a Global Clock
Before railways, every town kept its own solar time. Trains made that impossible, and within a few decades the world adopted a coordinated system anchored to the Greenwich meridian. This article traces the journey from local solar time through the 1884 International Meridian Conference to UTC.