code: public static String longToDateTime(long timestamp) {
          Date date = new Date(timestamp * 1000);
          DateFormat formatter = new SimpleDateFormat("yyyy.MM.dd HH:mm:ss");
          return formatter.format(date);
      }
docs: Return formatted Date String: yyyy.MM.dd HH:mm:ss Based on Unix's time()
  input in seconds
  
  @param timestamp
  seconds since start of Unix-time
  @return String formatted as - yyyy.MM.dd HH:mm:ss