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一年前，在网上找到的例子，现在已经找不到出处，对不住原作者，请原谅。修改了一下，在刷新页面的情况下，倒计时不重来。
<!DOCTYPE html>
<html>
<head lang="en">
<meta charset="UTF-8">
<title>倒计时</title>
</head>
<body>
<label id="time"></label>
<script>
function CountDown() {
var localtime=sessionStorage.getItem(‘time‘);
if(localtime){
if (localtime >= 0) {
hours=Math.floor(localtime/3600);
minutes = Math.floor((localtime-hours*3600)/ 60);
seconds = Math.floor((localtime-hours*3600) % 60);
hours = hours >= 10 ? hours : ‘0‘ + hours;
minutes = minutes >= 10 ? minutes : ‘0‘ + minutes;
seconds = seconds >= 10 ? seconds : ‘0‘ + seconds;
msg = "距离结束还有"+ hours + "时" + minutes + "分" + seconds + "秒";
document.getElementById(‘time‘).innerHTML = msg;
if (localtime == 5*60) alert(‘注意，还有5分钟!‘);
--localtime;
sessionStorage.setItem(‘time‘,localtime)
}
else {
document.getElementById(‘time‘).innerHTML = "时间到，结束!";
}
}else{
var maxtime = 90*60; //一个半小时，按秒计算，自己调整!
if (maxtime >= 0) {
hours=Math.floor(maxtime/3600);
minutes = Math.floor((maxtime-hours*3600)/ 60);
seconds = Math.floor((maxtime-hours*3600) % 60);
hours = hours >= 10 ? hours : ‘0‘ + hours;
minutes = minutes >= 10 ? minutes : ‘0‘ + minutes;
seconds = seconds >= 10 ? seconds : ‘0‘ + seconds;
msg = "距离结束还有"+ hours + "时" + minutes + "分" + seconds + "秒";
document.getElementById(‘time‘).innerHTML = msg;
--maxtime;
sessionStorage.setItem(‘time‘,maxtime)
}
else {
document.getElementById(‘time‘).innerHTML = "时间到，结束!";
}
}
}
timer = setInterval("CountDown()", 1000);
</script>
</body>
</html>
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原文地址：http://www.cnblogs.com/cyfhykx/p/5742823.html
