title: Adding <integer> seconds to a <timestamp> in PostgreSQL
body: <p>I have a table of items with the following columns:</p>
  
  <ul>
  <li><code>start_time</code> column (<code>timestamp without time zone</code>)</li>
  <li><code>expiration_time_seconds</code> column (<code>integer</code>)</li>
  </ul>
  
  <p>For example, some values are:</p>
  
  <pre><code>SELECT start_time, expiration_time_seconds 
     FROM whatever 
         ORDER BY start_time;
  
           start_time         | expiration_time_seconds
  ----------------------------+-------------------------
   2014-08-05 08:23:32.428452 |                  172800
   2014-08-10 09:49:51.082456 |                    3600
   2014-08-13 13:03:56.980073 |                    3600
   2014-08-21 06:31:38.596451 |                    3600
   ...
  </code></pre>
  
  <p>How do I add the expiration time, given in seconds, to the start_time? </p>
  
  <p>I have tried to format a time interval string for the <code>interval</code> command, but failed:</p>
  
  <pre><code>blah=&gt; SELECT interval concat(to_char(3600, '9999'), ' seconds');
  ERROR:  syntax error at or near "("
  LINE 1: SELECT interval concat(to_char(3600, '9999'), ' seconds');
  </code></pre>