HTML: Timer with Java Refresh

<html style="display:block; height:100%; margin:0; padding:0"
<head>
<style type="text/css">
    html {
        background: #000000;
        }
        #timer {
        padding: 100px;
        font-family: sans-serif;
        font-weight: bold;
        color: #FFFFFF;
        }
</style>
<meta http-equiv="refresh" content="1800">
</head>
<body style="display:table; width:100%; height:100%; margin:0; padding:0">

<div style="display:table-cell; width:100%; height:100%; text-align:center; vertical-align:middle">
    <span id="timer" style="font-size:400pt; font-kerning:none"></span>
</div>
<script type="text/javascript">
function checklength(i) {
    'use strict';
    if (i < 10) {
        i = "0" + i;
    }
    return i;
}

var minutes, seconds, count, counter, timer;
count = 1801; //seconds
counter = setInterval(timer, 1000);

function timer() {
    'use strict';
    count = count - 1;
    minutes = checklength(Math.floor(count / 60));
    seconds = checklength(count - minutes * 60);
    if (count < 0) {
        clearInterval(counter);
        return;
    }
    document.getElementById("timer").innerHTML = '' + minutes + ':' + seconds + ' ';
    if (count === 0) {
        location.reload();
    }
}
</script>
</body>
</html>