定時器是用以指定在一段特定的時間後執行某段程序。常用的定時器函數有以下幾個：
setTimeout()：定時器
clearTimeout()：終止定時器
setInterval()：設置定時器
clearInterval()：取消使用setInterval()設置的定時器
（1）系統時間向後走10秒即停止
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>無標題文檔</title>
<script>
var num=0;
var time;
function loadDate(){
var d=new Date();
var year=d.getYear();
var month=d.getMonth()+1;
var day=d.getDay()+1;
var hours=d.getHours();
var minutes=d.getMinutes();
var seconds=d.getSeconds();
var t=year+"/"+month+"/"+day+" "+hours+":"+minutes+":"+seconds;
var sp=document.getElementById("date");
sp.innerText=t;
num++;
if(num==10){
clearTimeout(time);
}
}
time=setInterval(loadDate,1000);
</script>
</head>
<body onload="loadDate()">
<span id="date"></span>
</body>
</html> （2）倒計時
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>無標題文檔</title>
<script>
var timmerID = null;
function time() {
var time = document.getElementById("now");
var left = document.getElementById("left");
time.value=parseInt(time.value)+1;
left.value = parseInt(left.value)-1;
if(left.value==0){
mystop();
alert("時間已到！");
}
}
function mystart() {
var left = document.getElementById("left");
left.value="10";
timmerID = window.setInterval("time()", 1000);
}
function mystop() {
window.clearInterval(timmerID);
}
</script>
</head>
<body>
時間起點：<input type="text" name="now" id="now" value="0"/><input type="button" value="開始" onclick="mystart();"><br>
剩餘時間：<input type="text" name="left" id="left" value="10" disabled="disabled"/>秒<br>
</body>
</html>