After reading about %z on these pages and http://docs.splunk.com/Documentation/Splunk/6.4.3/Data/Configuretimestamprecognition , I found the python 3 documentation https://docs.python.org/3/library/datetime.html has this explanation:
"
%z UTC offset in the form +HHMM or -HHMM (empty string if the object is naive). (empty), +0000, -0400, +1030 (6)
%Z Time zone name (empty string if the object is naive). (empty), UTC, EST, CST
"
The documentation has been prompt updated to reflect this information correctly!
Furthermore, the preview functionality mentioned is now part of the "add data"/data upload option in Splunk...
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