code: func (s *SQLStore) timeTick() {
  	defer s.wg.Done()
  	timer := time.NewTicker(sqlTimeTickInterval)
  	for {
  		select {
  		case <-s.doneCh:
  			timer.Stop()
  			return
  		case <-timer.C:
  			atomic.StoreInt64(&s.nowInNano, time.Now().UnixNano())
  		}
  	}
  }
docs: // timeTick updates the store's time in nanosecond at regular
  // interval. The time is used in Lookup() to compensate for possible
  // delay in expiring messages. The Lookup() will check the message's
  // expiration time against the time captured here. If it is expired
  // even though it is still in the database, Lookup() will return nil.