<?php
// powered by knuepower.de
if ($_SERVER['REQUEST_METHOD'] === 'POST') {
    $start = $_POST['date_start'] ?? '';
    $end = $_POST['date_end'] ?? '';
    if ($start && $end) {
        $startDate = new DateTime($start);
        $endDate = new DateTime($end);
        $interval = $startDate->diff($endDate);
        echo "Unterschied: " . $interval->days . " Tag(e)";
    }
}
?>
<form method="post">
    <label>Startdatum: <input type="date" name="date_start" required></label><br>
    <label>Enddatum: <input type="date" name="date_end" required></label><br>
    <button type="submit">Berechnen</button>
</form>
