javascript 到将来某个时间（2020-5-20）的倒计时

function countDown(dateStr){
    var end = +new Date(dateStr),
        start = +new Date(),
        during = Math.floor((end - start)/ 1000);

        var day = Math.floor(during/(24*60*60)),
            t1 = during - (day*24*60*60),
            hour = Math.floor(t1/(60*60)),
            t2 = t1- hour*60*60,
            min = Math.floor(t2/60),
            sec = t2%60;

        return day +'天' + hour + '小时' + min + '分' + sec + '秒';
}

setInterval(function(){
    var str = countDown('2020-11-23');
    document.write(str +"<br>");
}, 1000);