Leap seconds and celestial timestamps
Why UTC is not a uniform count of SI seconds and which bulletin must be retained for reproducible conversion.
Calculation contract · released 2026-08-18
Definition
A leap second changes the offset between UTC and atomic time so civil time remains close to Earth rotation.
Calculation procedure
Parse UTC with a versioned leap-second table, convert through TAI when required by the ephemeris path, and retain the bulletin or kernel version used.
Required inputs
- UTC instant
- Leap-second table or kernel version
- Target time scale
Maha convention
Maha fact bundles require a registered leap-second source even when the displayed chart is not sensitive at one-second scale.
Uncertainty and edge cases
Omitting or updating the table can shift high-precision state calculations; ordinary natal sign labels usually remain unchanged but reproducibility still breaks.
What this does not establish
Leap-second correctness does not improve an event timestamp that was only recorded to the minute or day.
Authoritative references
These sources establish the calculation vocabulary, data product, or transformation convention. They do not establish astrological interpretation.
- 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.
- 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.