code: func ToTTL(c clockwork.Clock, tm time.Time) time.Duration {
  	now := c.Now().UTC()
  	if tm.IsZero() || tm.Before(now) {
  		return 0
  	}
  	return tm.Sub(now)
  }
docs: // ToTTL converts expiration time to TTL duration
  // relative to current time as provided by clock