code: func correctedAge(h http.Header, reqTime, respTime time.Time) (time.Duration, error) {
  	date, err := timeHeader("Date", h)
  	if err != nil {
  		return time.Duration(0), err
  	}
  
  	apparentAge := respTime.Sub(date)
  	if apparentAge < 0 {
  		apparentAge = 0
  	}
  
  	respDelay := respTime.Sub(reqTime)
  	ageSeconds, err := intHeader("Age", h)
  	age := time.Second * time.Duration(ageSeconds)
  	correctedAge := age + respDelay
  
  	if apparentAge > correctedAge {
  		correctedAge = apparentAge
  	}
  
  	residentTime := Clock().Sub(respTime)
  	currentAge := correctedAge + residentTime
  
  	return currentAge, nil
  }
docs: // correctedAge adjusts the age of a resource for clock skew and travel time
  // https://httpwg.github.io/specs/rfc7234.html#rfc.section.4.2.3