用原生js做到固定时间的毫秒数

function gettime() {
获取当前时间的毫秒数
    var sTime= new Date().getTime();
获取截止时间的毫秒数
    var etime = "2018-04-26 16:00:00";
    var times = Date.parse(new Date(etime.replace(/-/g, "/")));
    var cha=times-sTime;
    var days    = cha / 1000 / 60 / 60 / 24;
    var daysRound   = Math.floor(days);
    var hours    = cha/ 1000 / 60 / 60 - (24 * daysRound);
    var hoursRound   = Math.floor(hours);
    var minutes   = cha / 1000 /60 - (24 * 60 * daysRound) - (60 * hoursRound);
    var minutesRound  = Math.floor(minutes);
    var secounds   = cha / 1000  - (24 * 60 * 60 * daysRound) - (60 * 60 *hoursRound) - (60*minutesRound);
    var secoundsRound  = Math.floor(secounds);
    if(secoundsRound<=9){
        secoundsRound="0"+secoundsRound;
    }
    document.getElementById("day").innerHTML=daysRound;
    document.getElementById("hour").innerHTML=hoursRound;
    document.getElementById("minutes").innerHTML=minutesRound;
    document.getElementById("secounds").innerHTML=secoundsRound;
}
window.οnlοad=function(){
    var that=this;
    that.gettime();
    setInterval(function () {
        that.gettime();

    },1000)}

效果图