The CDF_EPOCH routine was requested here: http://groups.google.com/group/comp.lang.idl-pvwave/browse_thread/thread/9e5ce62808a6028c#
There's a draft of it named NCDF_EPOCH in the CVS (NCDF_EPOCH function does not exist in IDL).
The CDF library is available from NASA here: http://cdf.gsfc.nasa.gov/
The CDF "copyright notice" seem to be compatible with GPL (?)
The CDF API manual (ftp://cdaweb.gsfc.nasa.gov/pub/cdf/doc/cdf33/cdf33crm.pdf) has a chapter on "EPOCH Utility Routines"