var year = 2021,
            month = 5,
            date = 19,
            sum = 0;
        switch(month) {
   
            case 12:
                sum += 31;
            case 11:
                sum += 30;
            case 10:
                sum += 31;
            case 9:
                sum += 30;
            case 8:
                sum += 31;
            case 7:
                sum += 30;
            case 6:
                sum += 31;
            case 5:
                sum += 30;
            case 4:
                sum += 31;
            case 3:
                sum += 28;
                if(year % 4 == 0 && year % 100 != 0 || year % 400 == 0) {
   //判断是否为闰年
                    sum += 1;
                }
            case 2:
                sum += 31;
            case 1:
                sum += date;
                break;
            default:
                alert("当前月份不存在");
                break;
        }
        alert("当前是今年的第" + sum + "天");