既然setTimeout是只执行一次，这里也没发现有循环的语句，他是怎么做到无限循环的

window.onload = function(){

showtime()

} 

function check(i){

if(i<10){

i = "0" +i;

}

return i;

}

function showtime(){

var curtime = new Date();

var endtime = new Date("2016/3/22,11:45:30");//?个重要

var lefttime = parseInt((endtime.getTime()-curtime.getTime())/1000)

var day = parseInt((lefttime)/(24*60*60));//Math不能漏

var date = parseInt(lefttime/(60*60)%24);

var mins = parseInt(lefttime/60%60);

var ss = parseInt(lefttime%60)

date = check(date);

mins = check(mins);

ss = check(ss);

document.getElementById("time").innerHTML = "距房子到期有　" + day + "天-" + date + ":" + mins +":" + ss+"";

if(lefttime<=0){

document.getElementById("time").innerHTML = "房子已到期"

}

var t = setTimeout(showtime,300);

}