title: Postgresql: how to correctly create timestamp with timezone from timestamp, timezone fields
body: <p>I have a table with a timestamp without time zone.
  YYYY-MM-DD HH:MM:SS</p>
  
  <p>and a field "timezone" that is either "P" for Pacific or "M" for Mountain.</p>
  
  <p>I need to create a field of type "timestamp with time zone"</p>
  
  <p>Given the two fields I have, is there a way to do this that correctly accounts for Daylight Saving Time?</p>
  
  <p>Specifically:
  timestamp: 2013-11-03 01:00:00
  timezone: "P"
  would become: 2013-11-03 01:00:00-07</p>
  
  <p>and 
  timestamp: 2013-11-03 03:00:00
  timezone: "P"
  would become: 2013-11-03 03:00:00-08</p>