code: func (tu Unit) Count(d time.Duration) (int, error) {
  	if d < 0 {
  		return 0, errNegativeDuraton
  	}
  
  	if dur, found := unitsToDuration[tu]; found {
  		return int(d / dur), nil
  	}
  
  	// Invalid unit.
  	return 0, errUnrecognizedTimeUnit
  }
docs: // Count returns the number of units contained within the duration.