javascript---按年/月/日的顺序输入一个任意日期，在页面输出：这是这一年的第几天

var year = parseInt(prompt("请输入年"));
var month = parseInt(prompt("请输入月"));
var day = parseInt(prompt("请输入日"));
var sum = 0;
switch(month - 1){
    case 11:
        sum += 30;
    case 10:
        sum += 31;
    case 9:
        sum += 30;
    case 8:
        sum += 31;
    case 7:
        sum += 31;
    case 6:
        sum += 30;
    case 5:
        sum += 31;
    case 4:
        sum += 30;
    case 3:
        sum += 31;
    case 2:
        //判断闰年  闰年2月29天
        if(((year % 4 == 0) && (year % 100 != 0)) || (year % 400 ==0)){
            sum += 29;
        }else {
            sum += 28;
        }
    case 1:
        sum += 31;
        break
}
sum += day;
document.write("这是" + year + "的第" + sum + "天");