title: How do you subtract from a datetime?
body: <p>I'd like to write a simple function to say in hours:</p>
  
  <p><strong>How much time has passed since this has been created?</strong></p>
  
  <p>My attempts:</p>
  
  <pre><code>-time = DateTime.now.hour - (self.created_at.hour)
  </code></pre>
  
  <p>Does anyone know how to do this in Ruby on Rails?</p>