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<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>Clone then set UTC ms</title>
</head>
<body>
  <h1>Immutable style</h1>
  <pre id="out"></pre>
  <script>
    var original = new Date(Date.UTC(2026, 2, 1, 12, 30, 45, 100));
    var adjusted = new Date(original);
    adjusted.setUTCMilliseconds(750);
    document.getElementById("out").textContent =
      "Original ms: " + original.getUTCMilliseconds() + "\n" +
      "Adjusted ms: " + adjusted.getUTCMilliseconds();
  </script>
</body>
</html>

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JavaScript Date setUTCMilliseconds() — clone first (Try It)