code: func checkLastModified(w http.ResponseWriter, r *http.Request, modtime time.Time) bool {
  	if modtime.IsZero() {
  		return false
  	}
  
  	// The Date-Modified header truncates sub-second precision, so
  	// use mtime < t+1s instead of mtime <= t to check for unmodified.
  	if t, err := time.Parse(http.TimeFormat, r.Header.Get("If-Modified-Since")); err == nil && modtime.Before(t.Add(1*time.Second)) {
  		h := w.Header()
  		delete(h, "Content-Type")
  		delete(h, "Content-Length")
  		w.WriteHeader(http.StatusNotModified)
  		return true
  	}
  	w.Header().Set("Last-Modified", modtime.UTC().Format(http.TimeFormat))
  	return false
  }
docs: // modtime is the modification time of the resource to be served, or IsZero().
  // return value is whether this request is now complete.