C Mini Calendar Project

I’ve been working on the mini calendar project for a while, but I’m just not understanding how to call functions correctly. I made it all the way to step 30, but right now my code just takes in the information and spits it back out without calling the add_days_to_date function. So I will type in a date and number of days to add and it will spit back out the date I entered. Here’s my code:

#include <stdio.h>
#include <stdbool.h>

//leap year bool

bool is_leap_year(int year){
if (year % 4 != 0) {
return false;
}
else if (year % 100 != 0) {
return true;
}
else if (year % 400 != 0) {
return false;
}
else {
return true;
}
}

//month int array
int days_in_month[13] = { 0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };

//days to add function;
void add_days_to_date(int* mm, int* dd, int* yy, int days_left_to_add)
{
int days_left_in_month;
int days_in_month;

while (days_left_to_add > 0)
{
	days_left_in_month == (days_in_month[mm] - *dd);
	if (*mm == 2 && is_leap_year(true))
	{
		days_left_in_month += 1;
	}
	if (days_left_to_add > days_in_month) 
	{
		days_left_to_add = days_left_to_add -= days_in_month + 1;
		*dd = 1;
		if (*mm == 12)
		{
			*mm = 1;
			*yy += 1;
		}
		else
		{
			*mm = *mm + 1;
		}
	}
	else
	{
		days_left_to_add = 0;
	}
}

}

int main() {

int mm, dd, yy, days_left_to_add;

printf("Enter a date between 1/1/1800 and 12/31/10000 and provide how many days you want to add: \n");
scanf("%i%i%i%i", &mm, &dd, &yy, &days_left_to_add);

add_days_to_date(&mm, &dd, &yy, days_left_to_add);
printf("%i/%i/%i", mm, dd, yy);
// int year;

//printf("Choose a year between 1800 and 10000: \n");
//scanf_s("%i", year);

//if (is_leap_year(year) == true) {
//	std::cout << "It is a Leap Year!";
//}
//else {
//std:cout << "It is NOT a Leap Year!";
//}

}

Anyways I’m pretty sure it’s an error with my printf() scanf() functions but I just cannot figure it out. Help would be much appreciated.