<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)
        });