Calculating days between two dates in C

Okay, so with the help of Alex and a few others for the past couple of days I managed to produce:

Code:

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


int check_and_assign_input(char *date, int *month, int *day, int *year)
{
    int slashcheck = 0;
    char *stepper = date;
    while(*stepper)
    {
        if(*stepper == '/')
        {
            slashcheck++;
            stepper++;
        }
        else stepper++;
    }


    if(slashcheck != 2)
        return 0;


  *month = atoi(strtok(date, "/"));
  *day =  atoi(strtok(NULL, "/"));
  *year = atoi(strtok(NULL, "/"));
        return 1;


}


int check_chron(int year1, int year2, int month1, int month2, int day1, int day2)
{
    if(year1 < year2)
        return 1;
    else if(year1 > year2)
        return 0;
    else if(year1 = year2)
    {
        if(month1 < month2)
            return 1;
        else if(month1 > month2)
            return 0;
        else if(month1 = month2)
        {
            if(day1 < day2)
                return 1;
            else if(day1 > day2)
                return 0;
            else if(day1 = day2)
            {
                printf("There are zero days between the two dates.");
                exit(1);
            }
        }
    }
}


int check_days_in_month(int month_check, int year_check)
{
    int days;
    switch(month_check)
    {
        case 1: case 3: case 5: case 7: case 8: case 10: case 12:
            days = 31;
            break;
        case 4: case 6: case 9: case 11:
            days = 30;
            break;
        case 2:
            if((year_check % 100 == 0) && (year_check % 400 != 0))
            {
                days = 28;
                break;
            }
            else if((year_check % 400 == 0) || (year_check % 4 == 0))
            {
                days = 29;
                break;
            }
            else
            {
                days = 28;
                break;
            }


    }


    return days;
}


int if_leap(int year_check)
{
    if((year_check % 100 == 0) && (year_check % 400 != 0))
        return 0;
    else if((year_check % 4) == 0)
        return 1;
    else
        return 0;
}


int calc_days(int year1, int year2, int month1, int month2, int day1, int day2, int days_in_month1, int days_in_month2)
{
    int days_until = 0;
    int month_days = 0;
    int days_in_year = 0;
    
    if(year1 < year2)
    {
        while(year1 < year2)
        {
            days_in_year = if_leap(year1);
            if(days_in_year == 1)
                days_until += 366;
            else days_until += 365;
            ++year1;
        }
        if(month1 < month2)
        {
            while(month1 < month2)
            {
                month_days = check_days_in_month(month1, year1);
                days_until += month_days;
                ++month1;
            }
        }
        if(month1 = month2)
        {
            month_days = check_days_in_month(month1, year1);
            days_until += (month_days - day2 - 5);
        }
    }


    else if(year1 = year2)
    {
        while(month1 < month2)
        {
            month_days = check_days_in_month(month1, year1);
            days_until += month_days;
            ++month1;
        }
        if(month1 = month2)
        {
            days_until += (day2 - day1);
        }
    }


    return days_until;
}


int main(void)
{
  char first_date[12];
  char second_date[12];
  int day1, month1, year1, day2, month2, year2;
  int days_in_month1, days_in_month2, chronology;
  int days_between, slashcheck;


  /* Get first date string */


  printf("Enter your first date: ");
  scanf("%s", first_date);

  slashcheck = check_and_assign_input(first_date, &month1, &day1, &year1);
  if(slashcheck == 0)
  {
      printf("Invalid input in first date.");
      exit(1);
  }  


  /*Call check_days_in_month function to test input day data against norm*/


  days_in_month1 = check_days_in_month(month1, year1);

  printf("Enter your second date: ");
  scanf("%s", second_date);


  slashcheck = check_and_assign_input(second_date, &month2, &day2, &year2);
  if(slashcheck == 0)
  {
      printf("\nInvalid input in second date.");
      exit(1);
  }  


  /*Call check_days_in_month function to test input day data against norm*/
  days_in_month2 = check_days_in_month(month2, year2);


  if((day1 > days_in_month1) || (day2 > days_in_month2))
  {
      printf("\nInvalid input day of first or second date.");
      exit(1);
  }
  
  chronology = check_chron(year1, year2, month1, month2, day1, day2);


  if(chronology == 0)
  {
      printf("\nInvalid input in chronology == 0.");
      exit(1);
  }


  days_between = calc_days(year1, year2, month1, month2, day1, day2, days_in_month1, days_in_month2);
  printf("\nThere are %d days between your two dates.", days_between);


    return 0;


}

I run into a problem whenever the dates input into the program are not in the same year, though, with the program usually being about 6 or more days over what the actual day count is.

I figure it has something to do with the algorithm of my calc_days function, but I can't seem to find the error.

If you can help in any way I would greatly appreciate it.

For ease of access, here's the calc_days function:

Code:

int calc_days(int year1, int year2, int month1, int month2, int day1, int day2, int days_in_month1, int days_in_month2){
    int days_until = 0;
    int month_days = 0;
    int days_in_year = 0;
    
    if(year1 < year2)
    {
        while(year1 < year2)
        {
            days_in_year = if_leap(year1);
            if(days_in_year == 1)
                days_until += 366;
            else days_until += 365;
            ++year1;
        }
        if(month1 < month2)
        {
            while(month1 < month2)
            {
                month_days = check_days_in_month(month1, year1);
                days_until += month_days;
                ++month1;
            }
        }
        if(month1 = month2)
        {
            month_days = check_days_in_month(month1, year1);
            days_until += (month_days - day2 - 5);
        }
    }


    else if(year1 = year2)
    {
        while(month1 < month2)
        {
            month_days = check_days_in_month(month1, year1);
            days_until += month_days;
            ++month1;
        }
        if(month1 = month2)
        {
            days_until += (day2 - day1);
        }
    }


    return days_until;
}