:param tzinfo: timezone, instance of datetime.tzinfo subclass
        :example '2003-10-21T16:05:52+0000'

def iso8601(self, tzinfo=None, end_datetime=None):
        """
        :param tzinfo: timezone, instance of datetime.tzinfo subclass
        :example '2003-10-21T16:05:52+0000'
        """
        return self.date_time(tzinfo, end_datetime=end_datetime).isoformat()