title: how can I calculate in serializer ?
body: <p>this is my serializer:<br>
  <strong>serializer</strong></p>
  
  <pre><code>start_at=serializers.DateTimeField(format="%Y-%m-%d %H:%M:%S")
  end_at = serializers.DateTimeField(format="%Y-%m-%d %H:%M:%S")  
  </code></pre>
  
  <p>now I want do calculate in serializer, make this <code>end_at - start_at</code><br>
  and give this value to <code>waite_time</code><br>
  how can I do this?</p>