count down timer javascript

<span id="countdown" class="timer"></span>
<?php
     
       $days_to_event = get_post_meta( get_the_ID(), 'days_to_event', true );  // days fetched from custom fields
        
       $final = $days_to_event*24*60*60;       
 
?>
<script>
var upgradeTime = <?php echo $final; ?>;
var seconds = upgradeTime;
function timer() {
    var days        = Math.floor(seconds/24/60/60);
    var hoursLeft   = Math.floor((seconds) - (days*86400));
    var hours       = Math.floor(hoursLeft/3600);
    var minutesLeft = Math.floor((hoursLeft) - (hours*3600));
    var minutes     = Math.floor(minutesLeft/60);
    var remainingSeconds = seconds % 60;
    if (remainingSeconds < 10) {
        remainingSeconds = "0" + remainingSeconds;
    }
    document.getElementById('countdown').innerHTML = days + "Days:" + hours + "Hours:" + minutes + "Minutes:" + remainingSeconds;
    if (seconds == 0) {
        clearInterval(countdownTimer);
        document.getElementById('countdown').innerHTML = "Completed";
    } else {
        seconds--;
    }
}
var countdownTimer = setInterval('timer()', 1000);
</script>