Extract the first date from 'key' matching YYYY-MM-DD
        or YYYY-MM, and convert to datetime.

def to_datetime(date_key):
        '''
        Extract the first date from 'key' matching YYYY-MM-DD
        or YYYY-MM, and convert to datetime.
        '''
        match = re.search(r'\d{4}-\d{2}(-\d{2})?', date_key)
        formatter = '%Y-%m'
        if len(match.group()) == 10:
            formatter += '-%d'
        return datetime.strptime(
            match.group(), formatter).replace(tzinfo=pytz.UTC)