计算两个日期相差多少天

#include<stdio.h>
#include<stdlib.h>

#define DayOfyear 365

#define ISLEAP(year) ((year % 4 = 0 && year % 100 != 0) || year % 400 == 0)

int isLeap(int year);
int dayOftheYear(int year, int month, int day);
int diff(int year1, int month1, int day1, int year2, int month2, int day2);
int main()
{
    int year1, month1,day1,
        year2,month2,day2;

    while(printf("input two dates:XXXX XX XX	 XXXX XX XX
"),scanf_s("%d %d %d %d %d %d",&year1,&month1,&day1,&year2,&month2,&day2) == 6)
    {
        printf("The days between the two dates is %d
",diff(year1,month1,day1,year2,month2,day2));
    }
    system("pause");
    return 0;
}
//判断闰年
int isLeap(int year)
{
    if((year % 4 == 0 && year % 100 != 0) || year % 400 == 0)
        return 1;//为闰年
    else
        return 0;//不是闰年
}

//计算某个日期在该年第多少天
int dayOftheYear(int year, int month, int day)
{
    int sum = 0;//记录天数
    int index_month;//遍历月份的下标
    if(month < 1 || month > 12 || day < 1 || day > 31)
    {
        printf("The date is illegal
");
        system("pause");
        return 0;
    }
    for(index_month = 1; index_month < index_month; index_month++)
    {
        switch(index_month)
        {
        case 1:
        case 3:
        case 5:
        case 7:
        case 8:
        case 10:
        case 12:
            sum += 31;
            break;
        case 4:
        case 6:
        case 9:
        case 11:
            sum += 30;
            break;
        case 2:
            if(isLeap(year) == 1)
                sum += 29;
            else
                sum += 28;
            break;
        }
    }
    sum += day;
    return sum;
}

int diff(int year1, int month1, int day1, int year2, int month2, int day2)
{
    int sum = 0;//记录两个日期的天数
    int index_year;//遍历年数
    int num_leap = 0;//记录闰年数

    for(index_year = year1; index_year < year2; index_year++)
    {
        if(isLeap(index_year) == 1)
            num_leap++;
    }
    sum = num_leap + (year2 - year1) * DayOfyear + dayOftheYear(year2,month2,day2) - dayOftheYear(year1,month1,day1);
    return sum;
}