<input id="countdown" type="text" value="http://www.mamicode.com/140时50分20秒">

<script>var oCountdown = document.getElementById("countdown"),    nowStamps = Date.now() - 100,            // 服务器当前时间    endStamps = Date.now() + 50000 - 100,    // 活动结束时间戳    diff = nowStamps - Date.now();            // 本地时间和服务器时间差function updateCountdown(nowStamps) {    var timeLeft = (endStamps - nowStamps)/1000,        hour = parseInt(timeLeft/3600),        minute = parseInt(timeLeft%3600/60),        seconds = parseInt(timeLeft%60);    oCountdown.value = hour + "时" + minute + "分" + seconds + "秒";    return timeLeft;}function run() {    nowStamps = Date.now() + diff;    // 服务器当前时间    if (updateCountdown(nowStamps) > 0) {    // 更新倒计时时间        setTimeout(run, 100);    // fix + 100 调整误差    } else {        oCountdown.value = "活动已结束";    }}setTimeout(run, 100);</script>