This calculates days between 2 dates need to find 100th day. I need to input the 1st date in the code.

#include<stdio.h> // include stdio.h library
#include<stdlib.h> // include stdlib.h library
int valid_date(int date, int mon, int year);

int main(void)
{
    int day1, mon1, year1,
        day2, mon2, year2;

    int day_diff, mon_diff, year_diff;

    printf("Enter start date (MM/DD/YYYY): ");
    scanf("%d/%d/%d", &mon1, &day1, &year1);

    printf("Enter end date (MM/DD/YYYY): ");
    scanf("%d/%d/%d", &mon2, &day2, &year2);

    if(!valid_date(day1, mon1, year1))
    {
        printf("First date is invalid.\n");
    }

    if(!valid_date(day2, mon2, year2))
    {
        printf("Second date is invalid.\n");
        exit(0);
    }

    if(day2 < day1)
    {
        // borrow days from february
        if (mon2 == 3)
        {
            //  check whether year is a leap year
            if ((year2 % 4 == 0 && year2 % 100 != 0) || (year2 % 400 == 0))
            {
                day2 += 29;
            }

            else
            {
                day2 += 28;
            }
        }

        // borrow days from April or June or September or November
        else if (mon2 == 5 || mon2 == 7 || mon2 == 10 || mon2 == 12)
        {
           day2 += 30;
        }

        // borrow days from Jan or Mar or May or July or Aug or Oct or Dec
        else
        {
           day2 += 31;
        }

        mon2 = mon2 - 1;
    }

    if (mon2 < mon1)
    {
        mon2 += 12;
        year2 -= 1;
    }

    day_diff = day2 - day1;
    mon_diff = mon2 - mon1;
    year_diff = year2 - year1;

    printf("Difference: %d years %02d months and %02d days.", year_diff, mon_diff, day_diff);

    return 0; // return 0 to operating system
}

// function to check whether a date is valid or not

int valid_date(int day, int mon, int year)
{
    int is_valid = 1, is_leap = 0;

    if (year >= 1800 && year <= 9999)
    {

        //  check whether year is a leap year
        if ((year % 4 == 0 && year % 100 != 0) || (year % 400 == 0))
        {
            is_leap = 1;
        }

        // check whether mon is between 1 and 12
        if(mon >= 1 && mon <= 12)
        {
            // check for days in feb
            if (mon == 2)
            {
                if (is_leap && day == 29)
                {
                    is_valid = 1;
                }
                else if(day > 28)
                {
                    is_valid = 0;
                }
            }

            // check for days in April, June, September and November
            else if (mon == 4 || mon == 6 || mon == 9 || mon == 11)
            {
                if (day > 30)
                {
                    is_valid = 0;
                }
            }

            // check for days in rest of the months
            // i.e Jan, Mar, May, July, Aug, Oct, Dec
            else if(day > 31)
            {
                is_valid = 0;
            }
        }

        else
        {
            is_valid = 0;
        }

    }
    else
    {
        is_valid = 0;
    }

    return is_valid;

}

I got this code in c that calculates day between 2 dates but I need to. modify it to find 1) 100th day. 2)I need to input the date today in the code.