And condense to one line...

<script>
var myVar=setInterval(function(){myTimer()},1000);

function myTimer()
{
document.body.innerHTML = "";
var unix = Math.round(+new Date() / 1000);
var year = Math.floor(unix / 38750400);
var yrem = unix % 38750400;
var day = Math.floor(yrem / 93600);
var drem = yrem % 93600;
var hour = Math.floor(drem / 3600);
var hrem = drem % 3600;
var min = Math.floor(hrem / 60);
var sec = hrem % 60;
document.write(year + ":" + day ":" + hour ":" + min ":" + sec);
}
</script>