Ephemeral znode will not be removed when sesstion timeout, if the system time of ZooKeeper node changes unexpectedly.

    @Override
    synchronized public void run() {
        try {
            while (running) {
                currentTime = System.currentTimeMillis();
                if (nextExpirationTime > currentTime) {
                    this.wait(nextExpirationTime - currentTime);
                    continue;
                }
                SessionSet set;
                set = sessionSets.remove(nextExpirationTime);
                if (set != null) {
                    for (SessionImpl s : set.sessions) {
                        setSessionClosing(s.sessionId);
                        expirer.expire(s);
                    }
                }
                nextExpirationTime += expirationInterval;
            }
        } catch (InterruptedException e) {
            handleException(this.getName(), e);
        }
        LOG.info("SessionTrackerImpl exited loop!");
    }