def _format_strings(self):
        """ we by definition have a TZ """

        values = self.values.astype(object)
        is_dates_only = _is_dates_only(values)
        formatter = (self.formatter or
                     _get_format_datetime64(is_dates_only,
                                            date_format=self.date_format))
        fmt_values = [formatter(x) for x in values]

        return fmt_values
we by definition have a TZ