code: def clean(self):
          
          cleaned_data = super(DatasetUploadForm, self).clean()
          date_begin = self.cleaned_data.get('date_begin')
          date_end = self.cleaned_data.get('date_end')
          if date_end < date_begin:
              msg = u'End date should be after start date.'
              self.add_error('date_begin', msg)
              self.add_error('date_end', msg)
          return cleaned_data
docs: Verifies that beginning date is before ending date.