HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>Half-hour reminder</title>
</head>
<body>
  <h1>Reminder check</h1>
  <pre id="out"></pre>
  <script>
    function checkHalfHourReminder() {
      var minute = new Date().getMinutes();
      if (minute === 0 || minute === 30) {
        return "Half-hour reminder!";
      }
      return "No reminder right now.";
    }

    document.getElementById("out").textContent = checkHalfHourReminder();
  </script>
</body>
</html>

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JavaScript Date getMinutes() — half-hour reminder (Try It)