Auto Subtract Time Zone Values

I'd like to automate changing all EST times to PST.  And this tool   —   http://indesignsecrets.com/free-script-to-change-all-numbers-in-a-docu ment-using-math.php  —  comes close.  There is a custom RegEx search param, but I cannot make it look at 1:45 and subtract the value to 10:45.  Maybe this is imposssible.