overwrite the attribute of time units

Hello.

I'm trying to change date information from original file.

This is the information of original file.

time ------> *time:units = "days since 0000-01-01 00:00:00" ;*

I want to change 0000-01-01 00:00:00 to 1990-01-01 00:00:00.

But, I failed to overwrite the time units.

How do I modified my program to change the time units?

==============================================
netcdf sst_HadOIBl_bc_0.47x0.63_clim_c061106_org {
dimensions:
        lon = 576 ;
        lat = 384 ;
        time = UNLIMITED ; // (12 currently)
variables:
        int date(time) ;
                date:long_name = "current date (YYYYMMDD)" ;
        int datesec(time) ;
                datesec:long_name = "current seconds of current date" ;
        double lon(lon) ;
                lon:long_name = "longitude" ;
                lon:units = "degrees_east" ;
        double lat(lat) ;
                lat:long_name = "latitude" ;
                lat:units = "degrees_north" ;
        double time(time) ;
              * time:units = "days since 0000-01-01 00:00:00" ;*
                time:calendar = "365_day" ;
        float ice_cov(time, lat, lon) ;
                ice_cov:long_name = "BCS Pseudo Sea-ice concentration" ;
                ice_cov:units = "fraction" ;

*
*
*Ncl code*
*=============================================================*
*
load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/contributed.ncl"

    begin

      in = addfile("./sst_HadOIBl_bc_0.47x0.63_clim_c061106_org.nc","w")

      a= in->time

      time = time_to_newtime(a, "days since 1999-01-01 00:00:00")

     printVarSummary(time)

    end

*

-- 
-- 
In-Won Kim
Intergrated Climate System Modeling Lab.
Department of Environmental Atmospheric Sciences
599-1 Daeyeon-dong, Nam-gu
Pukyong National University,
Busan, S.Korea
Tel. +82-51-629-6643
Fax +82-51-629-7991

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Received on Tue Jul 17 06:32:57 2012