searching recived header

So KMAIL will let me search all hederlines starting with "Received" with
a regex so I can find all my messages where any line marked Received
(could be 2 or could be 20) has a (PDT) or a (MST) for the Time zone but
its not a true regex search since $ [ ] ^ // seem meaningless to it what
I want is to find all email that originated outside of -600 (CST). Has
anyoner done this sort of thing with the KMAIL regex stuff?
Received on Tue Nov 27 16:53:00 2007