UTC, TAI, TT, TDB, and UT1
A concise map of the time scales that appear in civil records, ephemerides, Earth rotation, and reference-frame transformations.
Calculation contract · released 2026-08-18
Definition
UTC is civil time, TAI is atomic time, TT is terrestrial dynamical time, TDB is a barycentric coordinate time, and UT1 follows measured Earth rotation.
Calculation procedure
Declare the input scale, use versioned leap-second and Earth-orientation data for conversion, and supply the scale expected by each ephemeris or sidereal-time algorithm.
Required inputs
- Timestamp and stated scale
- Leap-second data
- Earth-orientation parameters where UT1 is needed
- Target algorithm contract
Maha convention
Maha stores the civil UTC instant separately from the ephemeris time scale and does not label all Julian dates as interchangeable.
Uncertainty and edge cases
The practical difference depends on the output: Earth-fixed angles need UT1, while high-precision solar-system states commonly use TT or TDB.
What this does not establish
Changing a time scale is a coordinate conversion, not an astrological technique.
Authoritative references
These sources establish the calculation vocabulary, data product, or transformation convention. They do not establish astrological interpretation.
- SPICE Time Subsystem
NASA/JPL Navigation and Ancillary Information Facility · 2021-12-31 revision
UTC, TAI, TT, TDB, calendar and Julian-date representations, leap-second kernels, and conversions used by SPICE.
Boundary: A civil timestamp is not an ephemeris time until its zone, offset, ambiguity resolution, leap-second data, and target time scale are declared.
- IERS Bulletins
International Earth Rotation and Reference Systems Service · live bulletin index
Bulletin A and B Earth-orientation parameters, Bulletin C leap-second announcements, and Bulletin D DUT1 announcements.
Boundary: The bulletin number or data-product version used in a computation must be retained; “latest” is not a reproducible version identifier.