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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>