code: func (r *Resource) Age() (time.Duration, error) {
  	var age time.Duration
  
  	if ageInt, err := intHeader("Age", r.header); err == nil {
  		age = time.Second * time.Duration(ageInt)
  	}
  
  	if proxyDate, err := timeHeader(ProxyDateHeader, r.header); err == nil {
  		return Clock().Sub(proxyDate) + age, nil
  	}
  
  	if date, err := timeHeader("Date", r.header); err == nil {
  		return Clock().Sub(date) + age, nil
  	}
  
  	return time.Duration(0), errors.New("Unable to calculate age")
  }
docs: // Calculate the age of the resource