See it in action on mamotok.org (you will be redirected back to ronaldpostma.com homepage)
<script>
var time = 15;
setInterval(function() {
var seconds = time % 60;
var minutes = (time - seconds) / 60;
if (seconds.toString().length == 1) {
seconds = "0" + seconds;
}
if (minutes.toString().length == 1) {
minutes = "0" + minutes;
}
document.getElementById("time").innerHTML = minutes + ":" + seconds;
time--;
if (time == 0) {
window.location.href = "https://www.ronaldpostma.com/";
}
}, 1000);
</script>
<div class="timer" onload="timer(20)">
<div class="time">
You will be redirected in <span id="time">'loading...'</span> seconds.
</div>
</div>