js:

setInterval(function () {
    var currentdate = new Date();
    var datetime = currentdate.getDate() + "/" + (currentdate.getMonth() + 1) + "/" + currentdate.getFullYear() + " @ " + currentdate.getHours() + ":" + currentdate.getMinutes() + ":" + currentdate.getSeconds();

    var target_matches = document.querySelectorAll('.target-timeout');
    target_matches[0].innerHTML = datetime;
}, 1000);

html:

<h3>update date and time every second</h3>
<h3 class="target-timeout"></h3>