code: func NewFileLogger(conf *FileConfig) (Logger, io.Closer, error) {
  	now := time.Now()
  	if conf.UTC {
  		now = now.UTC()
  	}
  
  	file, err := os.OpenFile(
  		timeutil.Strftime(&now, conf.Filename),
  		os.O_APPEND|os.O_CREATE|os.O_WRONLY, conf.FileMode)
  	if err != nil {
  		return nil, nil, err
  	}
  
  	logger, err := NewWriterLogger(conf.WriterConfig, file)
  	if err != nil {
  		file.Close()
  		return nil, nil, err
  	}
  
  	return logger, file, nil
  }
docs: // NewFileLogger creates a new file logger.