Counting days since 1.1.1

Question

So I have to write a program that counts day since 1.1.1 Additionally, I have to use Gregorian calendar for all this time (even though it wasn't used from 1.1.1) Anyways, here is what I wrote, but I am sure it has some mistakes:

#include <stdio.h>

int main()
{
    int d, m, r;
    printf("Podaj dzien, miesiac i rok\n");
    scanf("%d %d %d", &d, &m, &r);
    printf("%d", iledni(d, m, r));
    return 0;
}

int iledni(int d, int m, int r)
{
    int dni,x;
    int msc[12] = { 0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334 };
    dni = r * 365 + msc[m - 1] + d - 1;
    dni = dni + (int)((r - 1) / 4) - (int)((r - 1) / 100) + (int)((r - 1) / 400);
    if (m > 2 && r % 4 == 0 && r % 100 != 0 || r % 400 == 0)
        dni = dni + 1;
    return dni;
}

My next task is to write a function that counts days since my birth to current date, I am not sure how should the data be entered, so for now I assume it's just written in int main() so I have to change just the iledni function, but I have some problems with it, to be exact wrong result and no idea on how to fix it ><

#include <stdio.h>
int main()
{
    int d=17, m=8, r=2013;
    int du=25, mu=11, ru=1994;
    printf("%d", iledni(d, m, r, du, mu, ru));
    return 0;
}

int iledni(int d, int m, int r, int du, int mu, int ru)
{
    int dni,x;
    int msc[12]={0,31,59,90,120,151,181,212,243,273,304,334};
    int mscdodni[12]={31,28,31,30,31,30,31,31,30,31,30,31};
    dni=(r-ru)*365+msc[m-1]-msc[mu]+d-1+(mscdodni[mu-1]-du);
    for (ru=1994; ru<=r; ru++)
        {
        if (ru%4 == 0 && ru%100 != 0 || ru%400 == 0)
            dni=dni+1;
        }
return dni;
}

Tell me if I should explain any lines, also I am beginner so I am sorry if anything I wrote doesn't make sense, I don't know much about C or about programming as a whole.

Answer 1

Don't know if the following could be of any help, at least for the second part of your requirement (get the number of days from your birthdate to the current date):

This function will return the number of days from 1900-1-1:

int countDaysFrom1900(int y, int m, int d) {
    // The sum of days, equivalent to your msc array
    int days[] = {0, 31, days[1] + 28, days[2] + 31, days[3] + 30, days[4] + 31, days[5] + 30, days[6] + 31, days[7] + 31, days[8] + 30, days[9] + 31, days[10] + 30, days[11] + 31};

    // Offset the day value if the year is a leap year 
    // The: "(!(y % 400) || ((y % 100) && !(y % 4)))" thing is checking the leap year
    int offset = m > 2 && (!(y % 400) || ((y % 100) && !(y % 4))) ? 1 : 0;

    // Calculate the number of transcurred days 
    return y -= 1901, ((y + 1) * 365 + (y / 4) - (y / 100) + ((y + 1900) / 400) - 4 + days[m - 1] + offset + d - 1);
}

Here is an usage example:

int main() { 
    std::cout << countDaysFrom1900(1900, 1, 1) << std::endl;
    std::cout << countDaysFrom1900(1900, 1, 2) << std::endl;
    std::cout << countDaysFrom1900(1901, 1, 1) << std::endl;
    std::cout << countDaysFrom1900(2013, 10, 13) << std::endl;

    // Some birthdate
    int daysFromBirthdate = countDaysFrom1900(1982, 4, 27);
    int currentDate = countDaysFrom1900(2013, 10, 13);

    std::cout << "There are " << (currentDate - daysFromBirthdate) << " days from birthdate" << std::endl;
    return 0;
}

Prints:

0
1
365
41558
There are 11492 days from birthdate