A TimeFrame is a specialised form of one-dimensional Frame which represents various coordinate systems used to describe positions in time.
A TimeFrame represents a moment in time as either an Modified Julian Date (MJD), a Julian Date (JD), a Besselian epoch or a Julian epoch, as determined by the System attribute. Optionally, a zero point can be specified (using attribute TimeOrigin) which results in the TimeFrame representing time offsets from the specified zero point.
Even though JD and MJD are defined as being in units of days, the TimeFrame class allows other units to be used (via the Unit attribute) on the basis of simple scalings (60 seconds = 1 minute, 60 minutes = 1 hour, 24 hours = 1 day, 365.25 days = 1 year). Likewise, Julian epochs can be described in units other than the usual years. Besselian epoch are always represented in units of (tropical) years.
The TimeScale attribute allows the time scale to be specified (that is, the physical proces used to define
the rate of flow of time). MJD, JD and Julian epoch can be used to represent a time in any supported
time scale. However, Besselian epoch may only be used with the
" (Terrestrial Time) time scale.
The list of supported time scales includes universal time and siderial time. Strictly, these represent
angles rather than time scales, but are included in the list since they are in common use and are often
thought of as time scales.
When a time value is formatted it can be formated either as a simple floating point value, or as a Gregorian date (see the Format attribute).
When conversion between two TimeFrames is requested (as when supplying TimeFrames AST_CONVERT), account will be taken of the nature of the time coordinate systems they represent, together with any qualifying time scale, offset, unit, etc. The AlignSystem and AlignTimeScale attributes will also be taken into account.
A null Object pointer (AST__NULL) will be returned if this function is invoked with STATUS set to an error value, or if it should fail for any reason.