<h1 id="countdown"></h1>

 

var timer = null;

window.onload = function() {     timer = setInterval(clock, 1000); } var c = document.getElementById("countdown");

function clock() {

    var now = new Date();     var target = new Date("2999-1-25 3:00:00");     var time = (target - now) / 1000;     console.log(time)     if (time >= 0) {         var h = Math.floor(time / 60 / 60);         var m = Math.floor(time / 60 - (h * 60))         var s = Math.floor(time - (h * 60 * 60) - (m * 60))         h = h < 10 ? "0" + h : h         m = m < 10 ? "0" + m : m         s = s < 10 ? "0" + s : s         c.innerHTML = "距离时间还有" + h + "时" + m + "分" + s + "秒";     } else {         clearInterval(timer);         timer = null;         c.innerHTML = "";     }

}