As just a point of information, CDF Version 1 (as well as subsequent versions)
at NASA/GSFC-NSSDC has NO specific requirements or conventions for time. There
are specific CDF-based data analysis and visualization applications at NSSDC
that require a time convention for specific temporal manipulation utilities.
That convention is the one that Russ referenced. The specific choice of
"units" was based on the need to support historical data as well as space-borne
observations with a single "absolute" epoch reference. Nothing in CDF or
netCDF precludes the use of other time conventions. Many time-dependent data
sets have been created in CDF without the msec since 0 AD convention, or with
an additional temporal variable (e.g., a running time such as "time since
launch"). CDF as well as the NSSDC CDF utilities can handle those alternate
forms for time. It's just that there are specific utilities for that time
representation, which are optional for a user. I hope that clarifies the
point. I certainly have a number of ideas on time representation based
upon applications or storage form, which netCDF can easily support. But Russ'
approach is fine from the netCDF perspective. Specific netCDF-based software
may require something less flexible, but that's an exercise left up to the
netCDF user.
