SDF: Fractional months ?

[dan.swank]

Grads users:

I am creating a NetCDF file for a project demo.
my CDL time definition is:

        int t(t) ;
                t:long_name = "analysis time" ;
                t:units = "month since 2004-03-01" ;
                t:GRIB_orgReferenceTime = "2004-03-01T00:00:00Z" ;
                t:_CoordinateAxisType = "Time" ;

The data is 25 months of month-averaged GFS winds.
Which would in grads language be equivalent to:
tdef 25 linear 00z01mar2004 1mo

ncgen created the file no problem.  But grads don't like it:
"SDF:  Fractional months are ill-defined and not supported by GrADS"

Anyone know a quick reason?  Is this cdl time def. somehow not COORDS
complient?  What definition would work?

-Dan