title: How to add/subtract date/time components using a calculated interval?
body: <p>I'd like to get this to work in Teradata:</p>
  
  <p><strong>Updated SQL for better example</strong></p>
  
  <pre><code>select
  case
      when 
          current_date between
          cast('03-10-2013' as date format 'mm-dd-yyyy') and
          cast('11-03-2013' as date format 'mm-dd-yyyy')
      then 4
      else 5
  end Offset,
  (current_timestamp + interval Offset hour) GMT
  </code></pre>
  
  <p>However, I get an error of <code>Expected something like a string or a Unicode character blah blah</code>.  It seems that you have to hardcode the interval like this:</p>
  
  <pre><code>select current_timestamp + interval '4' day
  </code></pre>
  
  <p>Yes, I know I hardcoded it in my first example, but that was only to demonstrate a calculated result.</p>
  
  <p>If you must know, I am having to convert all dates and times in a few tables to GMT, but I have to account for daylight savings time.  I am in Eastern, so I need to add 4 hours if the date is within the DST timeframe and add 5 hours otherwise.</p>
  
  <p>I know I can just create separate update statements for each period and just change the value from a 4 to a 5 accordingly, but I want my query to be dynamic and smart.</p>