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I think I may have found the explanation (thanks to Eric Firing and
Sharon DeCarlo):<br>
<br>
From <a class="moz-txt-link-freetext" href="http://aa.usno.navy.mil/data/docs/JulianDate.php">http://aa.usno.navy.mil/data/docs/JulianDate.php</a><br>
<br>
The Julian date for CE 1999 July 20 00:00:00.0 UT is<br>
JD 2451379.500000<br>
The Julian date for CE 1 January 1 00:00:00.0 UT is<br>
JD 1721423.500000<br>
<br>
So the time of the QSCAT start since the year 1 origin is 729956
days. This agrees with <br>
the GrADS/GDS accounting.<br>
<br>
Eric Firing looked into all and summarized it here:<br>
<br>
<a class="moz-txt-link-freetext" href="http://matplotlib.org/api/dates_api.html">http://matplotlib.org/api/dates_api.html</a><br>
<br>
I suspect that what netCDF4 calls the "standard" calendar is this
USNO version that switches from Gregorian to Julian. <br>
<br>
<br>
Jim<br>
<br>
<div class="moz-cite-prefix">On 10/7/13 2:07 PM, Jennifer Adams
wrote:<br>
</div>
<blockquote
cite="mid:8367EC9F-1E8E-4538-987E-FBFFC0A496EB@cola.iges.org"
type="cite">Hi, Jim --
<div>Does the original descriptor file have a 365_day_calendar? </div>
<div>--Jennifer</div>
<div><br>
<div>
<div>On Oct 7, 2013, at 2:36 PM, James T. Potemra wrote:</div>
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<div bgcolor="#FFFFFF" text="#000000"> Hi all:<br>
<br>
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.<br>
<br>
Here's an example. The following is daily measurements
from QUIKSCAT:<br>
<meta http-equiv="content-type" content="text/html;
charset=ISO-8859-1">
<a moz-do-not-send="true" class="moz-txt-link-freetext"
href="http://apdrc.soest.hawaii.edu/dods/public_data/satellite_product/QSCAT/qscat_daily">http://apdrc.soest.hawaii.edu:80/dods/public_data/satellite_product/QSCAT/qscat_daily</a><br>
<br>
The data descriptor file has the following:<br>
TDEF 3776 linear 20JUL1999 1DY<br>
<br>
and the DDS page seems to have the correct values for
time:<br>
<meta http-equiv="content-type" content="text/html;
charset=ISO-8859-1">
<table>
<tbody>
<tr>
<td align="right" valign="top"><b>time</b>:</td>
<td align="left" valign="top">Array of 64 bit Reals
[time = 0..3775] <br>
grads_dim: "t" <br>
grads_mapping: "linear" <br>
grads_size: "3776" <br>
grads_min: "00z20jul1999" <br>
grads_step: "1dy" <br>
units: "days since 1-1-1 00:00:0.0" <br>
long_name: "time" <br>
minimum: "00z20jul1999" <br>
maximum: "00z19nov2009" <br>
resolution: 1.0 </td>
</tr>
</tbody>
</table>
<br>
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.<br>
<br>
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,<br>
<br>
Jim<br>
</div>
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<div>Jennifer M. Adams<br>
Center for Ocean-Land-Atmosphere Studies (COLA)<br>
111 Research Hall, Mail Stop 2B3<br>
George Mason University<br>
4400 University Drive<br>
Fairfax, VA 22030 <br>
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