code: def format_index(prefix, timestamp, sep='-'):
          
          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'))
docs: :type prefix str
          :type timestamp int
          :type sep str
          :rtype: str