Untitled

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text" id="time" style="border:3px solid blue; text-align:center; font-size:2em;" disabled="true"/>
<script>window.onload = function() {
    countdown(600);
}
function countdown(startTime) {
    let time = startTime;
    let box = document.getElementById('time');
    let intervalID = setInterval(decrement, 1000);

    function decrement() {
        time--;
        box.value = Math.trunc(time / 60) + ':' + ('0' + (time % 60)).slice(-2);
    }
}
</script>

</body>
</html>