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<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>Jan 31 UTC trap</title>
</head>
<body>
  <h1>Jan 31 → February</h1>
  <pre id="out"></pre>
  <script>
    var date = new Date(Date.UTC(2026, 0, 31));
    date.setUTCMonth(1);
    document.getElementById("out").textContent =
      "UTC month index: " + date.getUTCMonth() + "\n" +
      "UTC day: " + date.getUTCDate() + "\n" +
      "UTC year: " + date.getUTCFullYear();
  </script>
</body>
</html>

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