var xday = new Date(2007, 8, 30);
function countdown() {
    var now = new Date();
    var days = ((xday.getTime() - now.getTime())
                   / (24*60*60*1000));
    days = Math.ceil(days);
    var msg;
    if(days > 0) msg = "あと　<strong>" + days + 
                       "<\/strong>　日です。";
    else if(days == 0) msg = "今日から、 ****がはじまります。";
    else msg = "公演開始から　<strong>" + (-days) + 
               "<\/strong>　日経ちました。";
    document.write(msg);
}