<li id="time" style="display: none">3:00</li>
$(function () {
var all = $("#time").html();
var m = Number(all.substring(0, all.indexOf(":")));
var s = Number(all.substring(all.indexOf(":") + 1, all.length + 1));
var f = setInterval(function () {
if (s < 10) {
//如果秒数少于10在前面加上0
$('#time').html(m + ':0' + s);
} else {
$('#time').html(m + ':' + s);
}
s--;
if (s < 0) {
//如果秒数少于0就变成59秒
if (m == 0) {
window.clearInterval(f);
}
m--;
s = 59;
}
}, 1000)
});