Calendars and time
International Atomic Time
Direct answer
International Atomic Time: International Atomic Time, or TAI, is a continuous atomic time scale computed by the BIPM from clocks maintained by timing laboratories worldwide.
TAI is the international continuous atomic time scale maintained from an ensemble of clocks.
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At a glance
- Found in
- Calendars & Timekeeping
- Related
- Terrestrial Time, UT1, Calendar, Julian Date
TAI is the international continuous atomic time scale maintained from an ensemble of clocks.
A cycle or timeline schematic for comparing the named timekeeping concept.
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The story behind International Atomic Time
International clock comparisons and atomic standards enabled TAI to replace astronomical rotation as the most stable duration reference.
Why International Atomic Time matters
TAI provides a stable SI-second foundation for UTC, TT, navigation, telecommunications, and precision science.
International Atomic Time in practice: an example
UTC differs from TAI by an integer number of accumulated leap seconds.
Where readings of International Atomic Time go wrong
It is a mistake to call TAI a civil time zone; it has no daylight-saving or local-date rules.
Sources and editorial method
Research references used for this entry include:
- BIPM SI Brochure Annex: Time - International metrology reference for the SI second, TAI, UTC, and Terrestrial Time.
- NIST: UTC(NIST) Time Scale - Official description of UTC realization, atomic time, and leap-second coordination.
- Source
- The Great Almanac Dictionary
- Category
- Calendars & Timekeeping
- Last updated
- 2026-09-26
Trace atomic and civil time
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More in Calendars & Timekeeping
- Local Mean Solar Time
- Ordinal Date
- Sidereal Year
- Unix Time
- Astronomical Year Numbering
- Civil Calendar
- Draconic Year
- Golden Number
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Common questions
What is International Atomic Time?
International Atomic Time, or TAI, is a continuous atomic time scale computed by the BIPM from clocks maintained by timing laboratories worldwide.
What do people get wrong about International Atomic Time?
It is a mistake to call TAI a civil time zone; it has no daylight-saving or local-date rules.
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