title: add NSTimeInterval to NSDate in cocoa
body: <p>I've got a <code>NSDate</code> that represents a specific time. The format of that date is hhmmss. I want to add an <code>NSInterval</code> value (specified in seconds) to that time value.</p>
  
  <p>Example:</p>
  
  <pre><code>NSDate =         123000
  NSTimeInterval = 3600
  
  Added together = 133000
  </code></pre>
  
  <p>What is the best way to do this?</p>