title: Calculating difference in time between two Time objects
body: <p>Let's say I create two time objects from my user model:</p>
  
  <pre><code>created = u.created_at
  updated = u.updated_at
  </code></pre>
  
  <p>How do I calculate the difference in terms of hours between the two time objects?</p>
  
  <pre><code>hours = created - updated
  </code></pre>
  
  <p>I'd like to wrap this in a method and extend the Time class.  I find it hard to believe I'd need to extend it, but I can't seem to find a native method that handles calculating elapsed time using different time units.</p>