Un contador de tiempo para un evento en javascript

CPMexico countdown

<script type="text/javascript">
var millisecs_per_day=86400000
var evnt_name=""//Used during the event
var evnt_name_before=""//Used before the event
var evnt_name_after=""//Used after the event
var evnt_duration=7 //Event duration in days
var countdown_time=Date.UTC(2011,6,18, -5);//Year, mont, day. Dont know why the mont is minus 1.
var now=new Date();
var now_millisecs=now.valueOf();

var day_cnt= Math.ceil(( countdown_time - now_millisecs)/86400000 );

if ( day_cnt > 1 )
{
document.write("<div align='center'><strong>Faltan </strong><p><font size='+5'>" + day_cnt +"</font></p> días para <br />"+evnt_name_before+"</div>" );
}
else if ( day_cnt == 1 )
{
document.write("<div align='center'><strong>Faltan </strong><p><font size='+5'>" + day_cnt +"</font></p> días para <br />"+evnt_name_before+"</div>" );
}
else if ( day_cnt == 0 )
{
document.write("<p><div align='center'><font size='+2'>Hoy inicia</font><br />"+evnt_name+"</div></p>");
}
else if ( day_cnt > evnt_duration*-1 )
{
day_cnt-=1;
document.write("<p><div align='center'><font size='+2'>Día "+ day_cnt*-1 +" de</font><br />"+evnt_name+"</div></p>");
}
else if ( day_cnt == (evnt_duration*-1)-1 )
{
day_cnt-=1;
document.write("<p><div align='center'>"+evnt_name_after+"<br /> acabó ayer</div></p>");
}
else
{
day_cnt+=7;
document.write( "<p><div align='center'>"+evnt_name_after+"<br /> acabó hace "+ day_cnt*-1 +" días</div></p>");
}
</script>