title: Django Duration Field with negative value
body: <p>I have a model which contains a <code>DurationField</code>. If positive values are saved in this field, I get the correct result, if negative values are saved it returns "None" if I'm trying to access the attribute of the model.</p>
  
  <p>The model looks like the following:</p>
  
  <pre><code>class Calendar(models.Model):
      nominal_actual_comparison = models.DurationField(null=True,blank = True)
  </code></pre>
  
  <p>If I'm trying to access it now within a view like the following and the value is negative I'm obtaining a <code>NoneType</code> object:</p>
  
  <pre><code>calendar_object = Calendar.objects.get(id = 1)
  calendar_object.nominal_actual_comparison
  </code></pre>
  
  <p>I looked into the database and saw, that the <code>DurationField</code> is saved as a <code>BigInt</code>. The values in the database are definitely correct, therefore I'm wondering if there is a bug in the implementation of the <code>DurationField</code> or am I doing something wrong? What can I do instead? Is it possible to overwrite the <code>DurationField</code> class to adapt the way how the <code>BigInt</code> gets converted into a <code>datetime.timedelta</code> object? I saw a method called <code>to_python</code> which apparently calls a <code>parse_duration</code> method, but the <code>to_python</code> method is somehow never getting called !?
  I hope you can help me, thanks in advance!</p>