The Epoch attribute is stored as a Modified Julian Date, but when setting its value it may be given in a variety of formats. See the "Input Formats" section (below) for details. Strictly, the Epoch value should be supplied in the TDB timescale, but for some purposes (for instance, for converting sky positions between different types of equatorial system) the timescale is not significant, and UTC may be used.
The default Epoch value in a SkyFrame is B1950.0 (Besselian) for the old FK4-based coordinate systems (see the System attribute) and J2000.0 (Julian) for all others.
Care must be taken to distinguish the Epoch value, which relates to motion (or apparent motion) of the source, from the superficially similar Equinox value. The latter is used to qualify a coordinate system which is itself in motion in a (notionally) predictable way as a result of being referred to a slowly moving reference plane (e.g. the equator).
See the description of the System attribute for details of which qualifying attributes apply to each celestial coordinate system.
AST A Library for Handling World Coordinate Systems in Astronomy