title: iPhone: Convert date string to a relative time stamp
body: <p>I've got a timestamp as a string like:</p>
  
  <blockquote>
    <p>Thu, 21 May 09 19:10:09 -0700</p>
  </blockquote>
  
  <p>and I'd like to convert it to a relative time stamp like '20 minutes ago' or '3 days ago'.</p>
  
  <p>What's the best way to do this using Objective-C for the iPhone?</p>