title: TypeError: unsupported operand type(s) for -: 'DateTimeField' and 'DateTimeField'
body: <p>I have the following equation in a Django 2.1 class:</p>
  
  <pre><code>import datetime
  class FormBacktest(forms.Form):
       dateStart= forms.DateTimeField(label="Date Start", widget=DateTimePickerInput()
      dateEnd= forms.DateTimeField(label="Date End", widget=DateTimePickerInput()
      timeInMin = int((dateEnd-dateStart).total_seconds()/60)
  </code></pre>
  
  <p>I know from <a href="https://docs.djangoproject.com/en/2.1/ref/models/fields/#datetimefield" rel="nofollow noreferrer">the documentation</a> that the DateTimeField is a  datetime.datetime instance, so <a href="https://stackoverflow.com/questions/31156031/typeerror-unsupported-operand-types-for-datetime-time-and-datetime-time">this question</a> should be overcomed. </p>
  
  <p>Any recommendations?</p>