[gradsusr] GDS date-time standard

[James T. Potemra]

I think I may have found the explanation (thanks to Eric Firing and 
Sharon DeCarlo):

 From http://aa.usno.navy.mil/data/docs/JulianDate.php

The Julian date for CE  1999 July 20 00:00:00.0 UT is
JD 2451379.500000
The Julian date for CE     1 January  1 00:00:00.0 UT is
JD 1721423.500000

So the time of the QSCAT start since the year 1 origin is 729956 days. 
This agrees with
the GrADS/GDS accounting.

Eric Firing looked into all and summarized it here:

http://matplotlib.org/api/dates_api.html

I suspect that what netCDF4 calls the "standard" calendar is this USNO 
version that switches from Gregorian to Julian.


Jim

On 10/7/13 2:07 PM, Jennifer Adams wrote:
> Hi, Jim --
> Does the original descriptor file have a 365_day_calendar?
> --Jennifer
>
> On Oct 7, 2013, at 2:36 PM, James T. Potemra wrote:
>
>> Hi all:
>>
>> I'm wondering if anyone knows how GDS derives a date-time value from 
>> a GrADS style binary file?  For example, a contributed data set could 
>> have a descriptor file with a TDEF defining a start-date of Jan 1, 
>> 2013 and increment of 1dy.  If this file were served via GDS, the 
>> OPeNDAP date-time becomes "days-since-1-1-1".  In experimenting with 
>> some GDS-served files, it seems that this conversion is off by two days.
>>
>> Here's an example.  The following is daily measurements from QUIKSCAT:
>> http://apdrc.soest.hawaii.edu:80/dods/public_data/satellite_product/QSCAT/qscat_daily
>>
>> The data descriptor file has the following:
>> TDEF 3776  linear  20JUL1999  1DY
>>
>> and the DDS page seems to have the correct values for time:
>> *time*: 	Array of 64 bit Reals [time = 0..3775]
>> grads_dim: "t"
>> grads_mapping: "linear"
>> grads_size: "3776"
>> grads_min: "00z20jul1999"
>> grads_step: "1dy"
>> units: "days since 1-1-1 00:00:0.0"
>> long_name: "time"
>> minimum: "00z20jul1999"
>> maximum: "00z19nov2009"
>> resolution: 1.0
>>
>>
>> However, the OPeNDAP supplied first time-step is 729956 (obtained 
>> either through python call or ncdump).  I think that this corresponds 
>> to July 22, 1999 rather than the actual July 20, 1999 as defined in 
>> the descriptor file.
>>
>> Can anyone comment on what's going on here?  I found the same 
>> behavior with two other daily files, same apparent 2-day off-set.  
>> Thanks,
>>
>> Jim
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>
> --
> Jennifer M. Adams
> Center for Ocean-Land-Atmosphere Studies (COLA)
> 111 Research Hall, Mail Stop 2B3
> George Mason University
> 4400 University Drive
> Fairfax, VA 22030
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