title: DurationField format in a ModelForm
body: <p>I have a Django model that contains a duration field:</p>
  
  <pre><code>class Entry(models.Model):
      duration = models.DurationField()
  </code></pre>
  
  <p>And I want to render a form for this model using a ModelForm:</p>
  
  <pre><code>class EditEntryForm(forms.ModelForm):
      class Meta:
          model = Entry
          fields = ['duration']
  </code></pre>
  
  <p>Which is all working. However, if editing an existing model, the duration rendered in the text box is of the format <code>HH:MM:SS</code></p>
  
  <p>I will never be dealing with durations over an hour. How can I change how Django is formatting this field in the form to just be <code>MM:SS</code>?</p>
  
  <p>I already have a custom template filter in use when rendering the model, I just can't figure out how to change how the form is rendered.</p>
  
  <p>Thanks</p>