1-14作业

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<script>
    var n=prompt("请输入您的出生年份");
    var y=prompt("请输入你的出生月份");
    var r=prompt("请输入您的出生日期");
    var t=0;
    function Getday() {
        if (n%4 == 0){
            switch (y){
                case 1:
                t=t+r;
                break;
                case 2:
                t=31+r;
                break;
                case 3:
                t=60+r;
                break;
                case 4:
                t=91+r;
                break;
                case 5:
                t=121+r;
                break;
                case 6:
                t=152+r;
                break;
                case 7:
                t=182+r;
                break;
                case 8:
                t=213+r;
                break;
                case 9:
                t=244+r;
                break;
                case 10:
                t=274+r;
                break;
                case 11:
                t=305+r;
                break;
                case 12:
                t=335+r;
                break;
            }
        }return t;
    }
    Getday();
    document.write(n+"年第"+t+"天");
</script>
</body>
</html>
问题出在哪，日期输出一直是0