20050418: udunits documentation

Jeremy,

> Date: Thu, 14 Apr 2005 16:11:16 -0400
> From: Jeremy Rolph <address@hidden>
> To: address@hidden
> Subject: udunits documentation

The above message contained the following:

> Just curious, I am trying to use the udunits package and I am using
> PMEL TAO buoy data and I was comparing your reference date of
> 5/23/1968 00UTC as true Julian Day 2,400,000

I'm the author of the UDUNITS package and I'm I'm afraid I don't know
what you're talking about.  The date 1968-05-23 isn't a "reference date"
in the UDUNITS package (i.e., that date isn't used anywhere in the
package).  Also, the test program

    #include <stdio.h>
    #include <udunits.h>
    int
    main()
    {
        printf("Julian day number of 1968-05-23 is %ld\n",
            gregdate_to_julday(1968, 5, 23));
        return 0;
    }

prints

    Julian day number of 1968-05-23 is 2440000

rather than "2400000".

Perhaps you need to talk to the PMEL TAO buoy data people rather than
the UDUNITS package developer.

> to a date in one of the
> TAO netCDF files and simply subtracted the reference date from the
> dates in my file and I got a result of 51299.  I did some generic
> math and this results in a date roughly 140 yrs after 1968.  This
> obviously makes no sense.  I did more research and found this link on
> google.com:
> http://my.unidata.ucar.edu/content/software/netcdf/time/0057.html
> 
> On this page they are referencing 2,440,000 as 5/23/1968.  Which is
> correct.  If it is the second, 2,440,000, I think the documentation on the
> udunits pages should be updated to avoid further confusion.  Also, I
> should be using the msec as being since 00UTC rather than 12 UTC?
> 
> Let me know if I am wrong and which value is the correct one to use.
> 
> Thanks,
> Jeremy J. Rolph
> Center for Ocean Atmospheric Prediction Studies

Regards,
Steve Emmerson

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