code: func freshOCSP(resp *ocsp.Response) bool {
  	// start checking OCSP staple about halfway through validity period for good measure
  	refreshTime := resp.ThisUpdate.Add(resp.NextUpdate.Sub(resp.ThisUpdate) / 2)
  	return time.Now().Before(refreshTime)
  }
docs: // freshOCSP returns true if resp is still fresh,
  // meaning that it is not expedient to get an
  // updated response from the OCSP server.