public void clearExpiredSessions(){
		ArrayList toRemove = new ArrayList();
		long time_now = System.currentTimeMillis();
		
		for(int i = 0; i<sessions.size(); i++){
			if((time_now - sessions.get(i).last_time) > 360 * 1000)
				toRemove.add(i);
		}
		
		if(toRemove.size() > 0)
			sessions.removeAll(toRemove);
	}