code: def format_timestamp(ts):
          
          tz_info = tz.tzutc()
          return datetime.fromtimestamp(ts, tz=tz_info).strftime("%Y-%m-%dT%H:%M:%S.000Z")
docs: Format the UTC timestamp for Elasticsearch
          eg. 2014-07-09T08:37:18.000Z
          @see https://docs.python.org/2/library/time.html#time.strftime