code: protected File getLogFile(File parentLocation, long timestamp) {
          if (timestamp < 0) {
              throw new IllegalArgumentException("timestamp cannot be negative");
          }
          StringBuilder sb = new StringBuilder();
          sb.append(timestamp).append(EXTENSION);
          return new File(parentLocation, sb.toString());
      }
docs: calculates repository file name.
  
  @param timestamp the time in 'millis' of the first record in the file.
  @return the file according to repository pattern