code: func TTL(clock clockwork.Clock, t time.Time) time.Duration {
  	if t.IsZero() {
  		return Forever
  	}
  	diff := t.UTC().Sub(clock.Now().UTC())
  	if diff < 0 {
  		return Forever
  	}
  	return diff
  }
docs: // TTL converts time to TTL from current time supplied
  // by provider, if t is zero, returns forever