为什么我的代码在显示时，有时会在文本框内短暂地闪现time.getHours()+":"+time.getMinutes()+":" +time.getSeconds() 而不是一直显示时间
<!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;
}
setInterval("clock()",1000);
</script>
</head>
<body>
<form>
<input type="text" id="clock" size="50" />
</form>
</body>
</html>
