case 穿透 输入年月日判断这天是今天的第多少天

<input type="text" id="txt"/><br/>
<input type="button" id="btn" value="点击"/>
<script>

    btn.onclick=function(){
        var year = parseInt(txt.value/10000);
        var month = parseInt(txt.value/100)%100;
        var day = txt.value%100;
        if(year % 4 ==0 && year % 100 !=0 || year % 400 == 0){
            var d = 29;
        }else{
            var d = 28;
        }
        switch (month){
            case 12:
               day+=31;
            case 11:
                day+=30;
            case 10:
                day+=31;
            case 9:
                day+=30;
            case 8:
                day+=31;
            case 7:
                day+=31;
            case 6:
                day+=30;
            case 5:
                day+=31;
            case 4:
                day+=30;
            case 3:
                day+=31;
            case 2:
                day+=d;
            case 1:
                alert(day);
                break;
            default :
                alert('输入有误');
                break;
        }

    }
</script>