#include<stdio.h>
void main()
{
struct date
{int month
int day
int year
};date1;
int month_day,year_day,i;
printf("输入     月-日-年\n ");
scanf('%d-%d-%d",&date1.month,&date1.day,date.year);
for(i=1;i<date1.month;i++)
{switch(date1.month-i)
case 0:month_day=month_day+0;break;
case 1:month_day=month_day+31;break;
case 2:if(date1.year%400==0||(date1.year%4==0&&date1.year%100!=0))
{month_day=month_day+29;elsemonth_day=month_day+28;break;
case 3:month_day=month_day+31;break;
case 4:month_day=month_day+30;break;
case 5:month_day=month_day+31;break;
case 6:month_day=month_day+31;break;
case 7:month_day=month_day+31;break;
case 8:month_day=month_day+31;break;
case 9:month_day=month_day+30;break;
case 10:month_day=month_day+31;break;
case 11:month_day=month_day+30;break;
default:break;}
year_day=month_day+date1.day;
printf("%d",year_day);
}

struct date
{int month；
int day；
int year；
}此处无分号date1;
 month_day没有赋初值；并且你的swith思路感觉不对~