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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def starting_at(self, datetime_or_str):
 """ Set the starting time for the cron job. If not specified, the starting time will always be the beginning of the interval that is current when the cron is started. :param datetime_or_str: a datetime object or a string that dateutil.parser can understand :return: self """

        if isinstance(datetime_or_str, str):
            self._starting_at = parse(datetime_or_str)
        elif isinstance(datetime_or_str, datetime.datetime):
            self._starting_at = datetime_or_str
        else:
            raise ValueError('.starting_at() method can only take strings or datetime objects')
        return self