The following will convert from the timezone of your user account to UTC. You could use it in a subsearch to set earliest/latest in your search.
| makeresults
| eval myTime=_time
| eval showMyTime=strftime(myTime,"%H:%M")
| eval myZone=strftime(myTime,"%z")
| eval UTC=myTime-((myZone/100) *3600)
| eval showUTC=strftime(UTC,"%H:%M")
... View more