Try renaming your SplunkTime variable to _time in PowerShell and see if that works.
Splunk should pick up the time automatically, but what I can see from your output is that there are multiple times being returned, and that might lead to confusions.
That's the reason you need a props.conf stanza for your sourcetype where you specify things like your time format, prefix, lookahead, etc.
Take a look at this: http://docs.splunk.com/Documentation/Splunk/latest/Data/Configuretimestamprecognition
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