code: func (l *Logger) oldLogFiles() ([]logInfo, error) {
  	files, err := ioutil.ReadDir(l.dir())
  	if err != nil {
  		return nil, fmt.Errorf("can't read log file directory: %s", err)
  	}
  	logFiles := []logInfo{}
  
  	prefix, ext := l.prefixAndExt()
  
  	for _, f := range files {
  		if f.IsDir() {
  			continue
  		}
  		if t, err := l.timeFromName(f.Name(), prefix, ext); err == nil {
  			logFiles = append(logFiles, logInfo{t, f})
  			continue
  		}
  		if t, err := l.timeFromName(f.Name(), prefix, ext+compressSuffix); err == nil {
  			logFiles = append(logFiles, logInfo{t, f})
  			continue
  		}
  		// error parsing means that the suffix at the end was not generated
  		// by lumberjack, and therefore it's not a backup file.
  	}
  
  	sort.Sort(byFormatTime(logFiles))
  
  	return logFiles, nil
  }
docs: // oldLogFiles returns the list of backup log files stored in the same
  // directory as the current log file, sorted by ModTime