输出正确是第221天，但是显示输出错误 ，求大佬解释，

int sum;
    switch(month)
    {
        case 1:sum=0;break;
        case 2:sum=31;break;
        case 3:sum =59;break;
        case 4:sum =90;break;
        case 5:sum =120;break;
        case 6:sum =151;break;
        case 7:sum =181;break;
        case 8:sum =212;break;
        case 9:sum =243;break;
        case 10:sum =273;break;
        case 11:sum =304;break;
        case 12:sum =334;break;
    default:
        printf("一年只有12个月哦");
        break;
    }
    sum = sum+day;
        if((year%4==0 && year%100!=0)||(year%400==0))
        {
            sum += 1;
        }
        else
        sum +=0;
        printf("%d年%d月%d日这一天是该年的第%d天",year,month,day,sum);