window.onload = function() {
     
        var imgName = "newyear";//変更対象となる画像のID
        var newPass = "img/HappyNewYear2012.jpg";//変更する画像のパス
     
        var today=new Date();
        var newday = new Date(2012,0,1);
     
        var limit = new Date(2012,0,4);//期限
     
        if(today.getTime()>= newday.getTime() && today.getTime() <limit.getTime() ){
            document.getElementById(imgName).getAttributeNode("src").value= newPass;
        }
    }