code: func (i *Inspector) checkLifetime(c context.Context, body proto.Message) string {
  	lifespan := i.Lifespan(body)
  	now := clock.Now(c)
  	switch {
  	case lifespan.NotAfter == lifespan.NotBefore:
  		return "can't extract the token lifespan"
  	case now.Before(lifespan.NotBefore):
  		return "not active yet"
  	case now.After(lifespan.NotAfter):
  		return "expired"
  	default:
  		return ""
  	}
  }
docs: // checkLifetime checks that token has not expired yet.
  //
  // Returns "" if it hasn't expire yet, or an invalidity reason if it has.