title: Django - Format a calculated interval
body: <p><strong>Model</strong></p>
  
  <pre><code>class Session(models.Model):
      start_time = models.DateTimeField()
      end_time = models.DateTimeField()
      def duration(self):
          return self.end_time - self.start_time
  </code></pre>
  
  <p><strong>Template</strong></p>
  
  <pre><code>{{ session.duration|date:"H:i" }}
  </code></pre>
  
  <p>The duration can be displayed but seems not formattable. An idea why ?</p>