判断日期是今年的第几天

/* 计算天数 */
# include <stdio.h>

struct 
{
int year;
int month;
int day;
} data;    /* 定义一个结构并声明对象为data */

void main()
{
int days=0;
printf("请输入日期(年、月、日)：");
scanf("%d%d%d", &data.year, &data.month, &data.day);
switch(data.month)
{
case 1:  days = data.day;    
     break;
case 2:  days = data.day+31;
     break;
case 3:  days = data.day+59;
     break;
case 4:  days = data.day+90;
     break;
case 5:  days = data.day+120;
     break;
case 6:  days = data.day+151;
     break;
case 7:  days = data.day+181;