code: func parseOptInterval(f *Flag) (time.Duration, error) {
  	s := f.Value
  	if s == "" {
  		return 0, nil
  	}
  	d, err := time.ParseDuration(s)
  	if err != nil {
  		return 0, err
  	}
  	if d < container.MinimumDuration {
  		return 0, fmt.Errorf("Interval %#v cannot be less than %s", f.name, container.MinimumDuration)
  	}
  	return d, nil
  }
docs: // parseOptInterval(flag) is the duration of flag.Value, or 0 if
  // empty. An error is reported if the value is given and less than minimum duration.