This specifies the origin from which all time values are measured. The default value (zero) results in the TimeFrame describing absolute time values in the system given by the System attribute (e.g. MJD, Julian epoch, etc). If a TimeFrame is to be used to describe elapsed time since some origin, the TimeOrigin attribute should be set to hold the required origin value. The TimeOrigin value stored inside the TimeFrame structure is modified whenever TimeFrame attribute values are changed so that it refers to the original moment in time.

TimeFrame

The formats accepted when setting a TimeOrigin value are listed below. They are all case-insensitive and are generally tolerant of extra white space and alternative field delimiters:

• Besselian Epoch: Expressed in decimal years, with or without decimal places (" B1950" or " B1976.13" for example).

• Julian Epoch: Expressed in decimal years, with or without decimal places (" J2000" or " J2100.9" for example).

• Units: An unqualified decimal value is interpreted as a value in the system specified by the TimeFrame’ s System attribute, in the units given by the TimeFrame’ s Unit attribute. Alternatively, an appropriate unit string can be appended to the end of the floating point value (" 123.4 d" for example), in which case the supplied value is scaled into the units specified by the Unit attribute.

• Julian Date: With or without decimal places (" JD 2454321.9" for example).

• Modified Julian Date: With or without decimal places (" MJD 54321.4" for example).

• Gregorian Calendar Date: With the month expressed either as an integer or a 3-character abbreviation, and with optional decimal places to represent a fraction of a day (" 1996-10-2" or " 1996-Oct-2.6" for example). If no fractional part of a day is given, the time refers to the start of the day (zero hours).

• Gregorian Date and Time: Any calendar date (as above) but with a fraction of a day expressed as hours, minutes and seconds (" 1996-Oct-2 12:13:56.985" for example). The date and time can be separated by a space or by a " T" (as used by ISO8601 format).

When enquiring TimeOrigin values, the returned formatted floating point value represents a value in the TimeFrame’ s System, in the unit specified by the TimeFrame’ s Unit attribute.