title: How to add a time interval to an NSDate?
body: <p>I have an <code>NSDate</code> and a duration. I need to get the time after the duration</p>
  
  <p>Given:<br>
    The date is "2010-02-24 12:30:00 -1000"<br>
    duration is 3600 secs</p>
  
  <p>I need to get "2010-02-24 13:30:00 -1000"</p>
  
  <p>I thought <code>dateWithTimeIntervalSinceReferenceDate:</code>, would do the trick but I see now that this gives a date offset from 1 Jan 2001 GMT. </p>
  
  <p>Is there another C function I need to use</p>