Add hiddenUntil property for buttons

    <main>

        <h1>Was möchtest du tun?</h1>

        <a href="./test.html?id=practice" class="btn big">Linux üben</a>

        <a href="./test.html?id=pooltest" class="btn primary big">Den Pooltest ablegen</a>

        <a href="./test.html?id=pooltest" class="btn primary big" data-hidden-until="2022-10-15" display="none">Den Pooltest ablegen</a>

    </main>

    <div class="ribbon">Beta</div>

    <script type="module" src="./js/index.mjs"></script>

</body>

</html>

const now = Date.now()

document.querySelectorAll("[data-hidden-until]").forEach(element => {

    const hiddenUntil = Date.parse(element.dataset.hiddenUntil)

    element.style.display = now < hiddenUntil ? "none" : "inherit"

})