Julian Date conventions
How calendar timestamps become continuous day counts without losing calendar, timescale, or noon-origin assumptions.
Calculation contract · released 2026-08-18
Definition
Julian Date is a continuous day number whose integer boundary occurs at noon; Modified Julian Date uses a different offset and midnight boundary.
Calculation procedure
Convert a validated instant using a declared calendar convention and time scale, retain whether the value is JD or MJD, and avoid rounding before downstream calculations.
Required inputs
- Validated instant
- Time scale
- Gregorian/Julian calendar policy
- Requested JD representation
Maha convention
Maha treats a bare decimal labeled JD as incomplete unless its time scale and calendar conversion are known.
Uncertainty and edge cases
Confusing noon with midnight creates a half-day error; confusing UTC and TT introduces a smaller but systematic offset.
What this does not establish
A Julian Date is only a time coordinate and carries no interpretive meaning.
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.
- Horizons System Manual
NASA/JPL Solar System Dynamics · 4.98d (2025-11-21)
Solar-system object identifiers, observing centers, time inputs, reference frames, corrections, output quantities, and stated ephemeris limitations.
Boundary: Horizons output is authoritative only with the complete query contract preserved; transformed or rounded values require their own provenance record.