我想写比较两个日期先后的程序，但出现出乎意料的结果，就是不管哪一年的2月6号都会被认为是错误的，并且还不按我所说的错误处理方法printf("\nThe date does not exist!\tPLease input again!\n\n");
为此我还特意写了一个测试程序来验证，发现从0年到10000年的2月6号都会出现如此反常状况。

麻烦大神们帮我看一下



struct date
{
    int day;
    int month;
    int year;
}day[2];
int isleap(int year);            //判断是否闰年
int dateexist(struct date day);    //判断日期输入是否正确
main()
{
    int i=1,feb=0;
    while (i==1)
    {
        printf("Please input the date now:\n");
        printf("Day Month Year\n");
        scanf_s("%d %d %d", &day[0].day, &day[0].month, &day[0].year);
        i = dateexist(day[0]);
    }
    i = 1;
    printf("\n\n");
    while (i == 1)
    {
        printf("PLease input the date you want to:\n");
        printf("Day Month Year\n");
        scanf_s("%d %d %d", &day[1].day, &day[1].month, &day[1].year);
        i = dateexist(day[1]);
    }
    printf("\n\n");
    if (day[1].year > day[0].year)
    {
        printf("%d.%d.%d the future of %d.%d.%d.\n",day[1].month,day[1].day,day[1].year, day[0].month, day[0].day, day[0].year);
    }
    else if (day[1].year < day[0].year)
    {
        printf("%d.%d.%d the past of %d.%d.%d.\n", day[1].month, day[1].day, day[1].year, day[0].month, day[0].day, day[0].year);
    }
    else if (day[1].year == day[0].year)
    {
        if (day[1].month > day[0].month)
        {
            printf("%d.%d.%d the future of %d.%d.%d.\n", day[1].month, day[1].day, day[1].year, day[0].month, day[0].day, day[0].year);
        }
        else if (day[1].month < day[0].month)
        {
            printf("%d.%d.%d the past of %d.%d.%d.\n", day[1].month, day[1].day, day[1].year, day[0].month, day[0].day, day[0].year);
        }
        else if (day[1].month == day[0].month)
        {
            if (day[1].day > day[0].day)
            {
                printf("%d.%d.%d the future of %d.%d.%d.\n", day[1].month, day[1].day, day[1].year, day[0].month, day[0].day, day[0].year);
            }
            else if (day[1].day < day[0].day)
            {
                printf("%d.%d.%d the past of %d.%d.%d.\n", day[1].month, day[1].day, day[1].year, day[0].month, day[0].day, day[0].year);
            }
            else if (day[1].day == day[0].day)
            {
                printf("It's today.\n");
            }
        }
    }
    getchar();
    getchar();
}
int isleap(int year)
{
    int judge;
    judge = ((year % 172800 == 0) || (year % 4 == 0 && year % 100 != 0) || (year % 1000 == 0 && year % 400 == 0)) ? 1 : 0;
    return judge;
}
int dateexist(struct date day)
{
    int i=0,feb=0;
    i = (day.month > 0 && day.month < 13) ? 0 : 1;
    if (isleap(day.year))
    {
        feb = 29;
        printf("Leap Year\n");
    }
    else
    {
        feb = 28;
        printf("Common Year\n");
    }
    if (i == 0)
    {
        switch (day.month)
        {
        case 1:
        case 3:
        case 5:
        case 7:
        case 8:
        case 10:
        case 12:
            i = (day.day <= 0 || day.day > 31) ? 1 : 0;    break;
        case 4:
        case 6:
        case 9:
        case 11:
            i = (day.day <= 0 || day.day > 30) ? 1 : 0;    break;
        case 2:
            i = (day.day <= 0 || day.day > feb) ? 1 : 0; break;
        }
    }
    if (i == 1)
        printf("\nThe date does not exist!\tPLease input again!\n\n");
}

不好意思了，是因为下面的dateexist函数没有写返回值，i=1后应该返回i的，新手忘了，现在可以正常运行了，大家有兴趣的话帮忙看看有什么可以改进的地方吧！谢谢！