code: def _set_date_tuple(self, mediafile, year, month=None, day=None):
          
          if year is None:
              self.__delete__(mediafile)
              return
  
          date = [u'{0:04d}'.format(int(year))]
          if month:
              date.append(u'{0:02d}'.format(int(month)))
          if month and day:
              date.append(u'{0:02d}'.format(int(day)))
          date = map(six.text_type, date)
          super(DateField, self).__set__(mediafile, u'-'.join(date))
  
          if hasattr(self, '_year_field'):
              self._year_field.__set__(mediafile, year)
docs: Set the value of the field given a year, month, and day
          number. Each number can be an integer or None to indicate an
          unset component.