public String toHowLong() {
		String ret="";
		ret+=day==0? "": day + "日";
		ret+=hour==0? "": hour + "時間";
		ret+=minute==0? "": minute + "分";
		ret+=second==0? "": second + "秒";
		if(ret.length()==0)
			ret="0秒";
		return ret;
	}