I was able to convert the PT timestamps to UTC (time_utc) via the following eval:
eval time_splunk=strftime(_time, "%A, %B %e, %Y %l:%M:%S.%3Q %p %Z (%:z)") | eval time_offset=strftime(_time, "%:z") | rex field=time_offset ".(?<time_offset_seconds>\d{2}:\d{2})" | eval time_offset_seconds=time_offset_seconds.":00" | convert dur2sec(time_offset_seconds) | eval time_utc_epoch=strftime(_time, "%s") | convert num(time_utc_epoch) | eval time_utc_epoch=if(time_offset_seconds==0, time_utc_epoch, if(substr(time_offset, 1, 1)=="+", time_utc_epoch-time_offset_seconds, time_utc_epoch+time_offset_seconds)) | eval time_utc=strftime(time_utc_epoch, "%Y-%m-%dT%H:%M:%SZ")
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