Where do you use this SPL? Are you building a new panel in a dashboard?
If this is your case, you can add a timepicker and link your panel with it.
But anyway, you can restrict your search like this:
index=servicenow eventtype=snow_change* sourcetype="snow:change_request" (change_state_name="Work Complete" OR change_state_name=Closed) earliest=-30d latest=now | dedup .......
More info and options here:
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