:type prefix str
        :type timestamp int
        :type sep str
        :rtype: str

def format_index(prefix, timestamp, sep='-'):
        """
        :type prefix str
        :type timestamp int
        :type sep str
        :rtype: str
        """
        tz_info = tz.tzutc()

        # ex. logstash-other-2017.05.09
        return "{prefix}{sep}{date}".format(
            prefix=prefix, sep=sep, date=datetime.fromtimestamp(timestamp, tz=tz_info).strftime('%Y.%m.%d'))