如果点击开始按钮进行setinterval 点击取消按钮clearinterval clearinterval（）中返回的值怎么获得
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Title</title>
<script>
function start() {
var today = new Date();
document.getElementById("txt").innerHTML=today.getHours()+":"+today.getMinutes()+":"+today.getSeconds();
i=setInterval("start()",1000)
}
function stop() {
clearInterval(i);
}
</script>
</head>
<body>
<button onclick="start()">单击这里</button>
<button onclick="stop()">单击这里</button>
<div id="txt" ></div>
</body>
</html>
