为什么代码正确但是文本框却没有内容？
<!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.getElementById("clock").value = attime;
}
window.setInterval(clock(),1000)
</script>
</head>
<body>
<form>
<input type="text" id="clock" size="50"/>
</form>
</body>
</html>
