Implements date string parsing adhering to RFC 6265.

def _parse_date(cls, date_str: str) -> Optional[datetime.datetime]:
        """Implements date string parsing adhering to RFC 6265."""
        if not date_str:
            return None

        found_time = False
        found_day = False
        found_month = False
        found_year = False

        hour = minute = second = 0
        day = 0
        month = 0
        year = 0

        for token_match in cls.DATE_TOKENS_RE.finditer(date_str):

            token = token_match.group("token")

            if not found_time:
                time_match = cls.DATE_HMS_TIME_RE.match(token)
                if time_match:
                    found_time = True
                    hour, minute, second = [
                        int(s) for s in time_match.groups()]
                    continue

            if not found_day:
                day_match = cls.DATE_DAY_OF_MONTH_RE.match(token)
                if day_match:
                    found_day = True
                    day = int(day_match.group())
                    continue

            if not found_month:
                month_match = cls.DATE_MONTH_RE.match(token)
                if month_match:
                    found_month = True
                    month = month_match.lastindex
                    continue

            if not found_year:
                year_match = cls.DATE_YEAR_RE.match(token)
                if year_match:
                    found_year = True
                    year = int(year_match.group())

        if 70 <= year <= 99:
            year += 1900
        elif 0 <= year <= 69:
            year += 2000

        if False in (found_day, found_month, found_year, found_time):
            return None

        if not 1 <= day <= 31:
            return None

        if year < 1601 or hour > 23 or minute > 59 or second > 59:
            return None

        return datetime.datetime(year, month, day,
                                 hour, minute, second,
                                 tzinfo=datetime.timezone.utc)