main.js
'use strict';
{
// 2000 4 11
// const d = new Date(2000, 3, 11);
// 2000 2 ??
const d = new Date(2000, 3, 0);
d.setDate(d.getDate() + 100);
console.log(d.toLocaleString());
}index.html
<!DOCTYPE html>
<html lang="ja">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>My JavaScript</title>
</head>
<body>
<script src="main.js"></script>
</body>
</html>