def parse_date_str_series(format_str, tz, date_str_series, data_frequency,
                              trading_day):
        """
        Efficient parsing for a 1d Pandas/numpy object containing string
        representations of dates.

        Note: pd.to_datetime is significantly faster when no format string is
        passed, and in pandas 0.12.0 the %p strptime directive is not correctly
        handled if a format string is explicitly passed, but AM/PM is handled
        properly if format=None.

        Moreover, we were previously ignoring this parameter unintentionally
        because we were incorrectly passing it as a positional.  For all these
        reasons, we ignore the format_str parameter when parsing datetimes.
        """

        # Explicitly ignoring this parameter.  See note above.
        if format_str is not None:
            logger.warn(
                "The 'format_str' parameter to fetch_csv is deprecated. "
                "Ignoring and defaulting to pandas default date parsing."
            )
            format_str = None

        tz_str = str(tz)
        if tz_str == pytz.utc.zone:
            parsed = pd.to_datetime(
                date_str_series.values,
                format=format_str,
                utc=True,
                errors='coerce',
            )
        else:
            parsed = pd.to_datetime(
                date_str_series.values,
                format=format_str,
                errors='coerce',
            ).tz_localize(tz_str).tz_convert('UTC')

        if data_frequency == 'daily':
            parsed = roll_dts_to_midnight(parsed, trading_day)
        return parsed