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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 _month_converter(self, date_time):
 """Returns Billing API format YYYY-DD"""

        if not date_time:
            date_time = datetime.datetime.utcnow()
        if isinstance(date_time, datetime.datetime):
            date_time = '%s-%02d' % (date_time.year, date_time.month)
        return date_time