code: func useCached(downloadTime time.Time, maxAge int) bool {
  	freshnessLifetime := int(time.Now().Sub(downloadTime).Seconds())
  	if maxAge > 0 && freshnessLifetime < maxAge {
  		return true
  	}
  	return false
  }
docs: // useCached checks if downloadTime plus maxAge is before/after the current time.
  // return true if the cached image should be used, false otherwise.