请问 我的代码有何问题？
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>定时器</title>
<script type="text/javascript">
var attime;
function clock(){
var time=new Date();
attime = time.getHours()+":"+time.getMinutes()+":"+time.getSeconds() ;
// document.write(attime);
document.getElementById("clock").value = attime;
}
setInterval(clock(),1000);
</script>
</head>
<body>
<form>
<input type="text" id="clock" size="50" />
</form>
</body>
</html>浏览器出错提示是 ：Uncaught TypeError: Cannot set property 'value' of null 第11行 document.getElementById("clock").value = attime;
