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<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>Jan 31 to Feb</title>
</head>
<body>
  <h1>Jan 31 → setMonth(1)</h1>
  <pre id="out"></pre>
  <script>
    var date = new Date(2026, 0, 31);
    date.setMonth(1);
    document.getElementById("out").textContent =
      "Month index: " + date.getMonth() + "\n" +
      "Day: " + date.getDate() + "\n" +
      "Year: " + date.getFullYear();
  </script>
</body>
</html>

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JavaScript Date setMonth() — Jan 31 overflow (Try It)