大佬们，这括号里81是什么意思
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>获得年份 </title>
<script type="text/javascript">
var mydate=new Date();
document.write(mydate+"<br>");
document.wirte(mydate.getFullYear()+"<br>");
mydate.setFullYear(81);
var myyear= mydata.getFullYear(81);
document.write("年份:"+myyear);
</script>
</head>
<body>
</body>
</html>
