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);