## Specifying a Particular Time System

By setting the SystemSystem attribute appropriately, the TimeFrameTimeFrame can represent Julian Date, Modified Julian Date, Julian EpochEpoch or Besselian Epoch (the time scale is specified by a separate attribute called TimeScaleTimeScale).

Selection of a particular coordinate system is performed simply by setting a value for the TimeFrame's (character string) System attribute. This setting is most conveniently done when the TimeFrame is created. For example, a TimeFrame representing Julian Epoch would be created by:

180#180

Note that specifying “System154#154JEPOCH” also changes the associated default Unit (from days to years). This is because the default value of the TimeFrame's Unit attribute depends on the System attribute setting.

You may change the System value at any time, although this is not usually needed. The values supported are set out in the attribute's description in Appendix C.