代码如下：
#include <stdio.h>
void main()
{
    int year1,month1,date1,days1=0,year2,month2,date2,days2=0,n,m,k,days;
    printf("请输入出生年月日:\n");
    scanf("%d%d%d",&year1,&month1,&date1);
    printf("请输入计算天的年月日:\n");
    scanf("%d%d%d",&year2,&month2,&date2);
    n=year1;
    if(year1<=year2-1)
    {
        for(n=year1,days=0;n<=year2-1;n++) //统计year1到year2-1的天数
        {   
            if(n%4==0 && n%100!=0 || n%400==0)
                days+=366;
            else  days+=365;
        }
    }
    else days=0;
    printf("year1到year2-1的天数:  %d\n",days);
    for(m=1;m<=month1-1;m++)//计算出出生日是当年的days1天
    {
        if(n%4==0 && n%100!=0 || n%400==0)
        {
            if(m==1 || m==3 || m==5 || m==7 || m==8 || m==10 || m==12)
                days1+=31;
            else if(m==2)
                days1+=29;
            else days1+=30;
        }
        else
        {
            if(m==1 || m==3 || m==5 || m==7 || m==8 || m==10 || m==12)
            days1+=31;
            else if(m==2)
            days1+=28;
            else days1+=30;
        }
    }
     days1+=date1;

    for(k=1;k<=month2-1;k++)//计算出计算日是当年的days2天
    {
        if(year2%4==0 && year2%100!=0 || year2%400==0)
        {
            if(k==1 || k==3 || k==5 || k==7 || k==8 || k==10 || k==12)
                days2+=31;
            else if(k==2)
                days2+=29;
            else days2+=30;
        }
        else
        {
            if(k==1 || k==3 || k==5 || k==7 || k==8 || k==10 || k==12)
            days2+=31;
            else if(k==2)
            days2+=28;
            else days2+=30;
        }
    }
    days2+=date2;
    printf("%d\n",days+days2-days1);
}

是的，有些运行结果是不正确的

懂了，谢谢各位，我忘记n=year1在执行之后会n变化的