C Programs for Calendar (DIsplay Month and Year)

This is the code I have so far:

Code:

---

#include <stdio.h>

char *months[] =
    {
    " ",
    "\nJanuary",
    "\nFebruary",
    "\nMarch",
    "\nApril",
    "\nMay",
    "\nJune",
    "\nJuly",
    "\nAugust",
      "\nSeptember",
    "\nOctober",
    "\nNovember",
    "\nDecember"
    };

int month_days[] = {0, 31, 28, 31, 30, 31, 30, 31 ,31 ,30, 31, 30, 31};

int first_day_year(int year)
{
    int first_day;
    int x;
    int y;
    int z;

    first_day = (year + x - y + z) %7;

    return first_day;
}

int leapyear(int year)
{
    if(year%4 == 0 && year%100 != 0 || year%400 == 0)
    {
        month_days[2] = 29;
        return 1;
    } else {
        month_days[2] = 28;
        return 0;
    }
}

int calendar(int month, int year, int first_day)
{
    int i;

    printf("%s %d\n\n", months[month], year);
    printf("Sun  Mon  Tue  Wed  Thu  Fri  Sat\n");

    for(i = 0; i < month; i++)
    {
        first_day = (first_day + month_days[month]) % 7;
    }

    for(i = 1; i <= 1 + first_day * 5; i++)
    {
        printf(" ");
    }

    for(i = 1; i <= month_days[month]; i++)
    {
        printf("%2d", i);

        if((i + first_day)%7 > 0)
            printf("  ");
        else
            printf("\n ");
    }
}

int main(void)
{
    int year;
    int month;
    int first_day;

    do{
        printf("Enter a month (1 - 12): ");
        scanf("%d", &month);
    } while (month < 1 || month > 12);

    do{
        printf("Enter a year (1978 - 3000): ");
        scanf("%d", &year);
    } while (year < 1978 || year > 3000);

    first_day = first_day_year(year);
    leapyear(year);
    calendar(month, year, first_day);
   
    printf("\n");

    return 0;
}

---

The program basically asks for a month and year and then displays the calendar for the input.
I'm able to compile this code and it works fine for March 2012, but apparently, in other months, the day starts a day before it originally should.

For example, January 1978 starts on a Sunday, but when I run my program, it starts on a Saturday.

How can I fix this?
Please help!

x,y and z are uninitialized in your first_day_year() function. Don't know what you expect of this but it will not give you a reliable answer.

@Subsonics, yup. Saw it immediately after I posted this. But even though I fixed it, the program gave even weirder outputs.

Here's my code now:

Code:

---

#include <stdio.h>

char *months[] =
    {
    " ",
    "\nJanuary",
    "\nFebruary",
    "\nMarch",
    "\nApril",
    "\nMay",
    "\nJune",
    "\nJuly",
    "\nAugust",
      "\nSeptember",
    "\nOctober",
    "\nNovember",
    "\nDecember"
    };

int month_days[] = {0, 31, 28, 31, 30, 31, 30, 31 ,31 ,30, 31, 30, 31};

int first_day_year(int year)
{
    int first_day;
    int x;
    int y;
    int z;

    x = (year - 1.)/4.0;
    y = (year - 1.)/100.;
    z = (year - 1.)/400.;

    first_day = (year + x - y + z) %7;

    return first_day;
}

int leapyear(int year)
{
    if(year%4 == 0 && year%100 != 0 || year%400 == 0)
    {
        month_days[2] = 29;
        return 1;
    } else {
        month_days[2] = 28;
        return 0;
    }
}

int calendar(int month, int year, int first_day)
{
    int i;

    printf("%s %d\n\n", months[month], year);
    printf("Sun  Mon  Tue  Wed  Thu  Fri  Sat\n");

    for(i = 0; i < month; i++)
    {
        first_day = (first_day + month_days[month]) % 7;
    }

    for(i = 1; i <= 1 + first_day * 5; i++)
    {
        printf(" ");
    }

    for(i = 1; i <= month_days[month]; i++)
    {
        printf("%2d", i);

        if((i + first_day)%7 > 0)
            printf("  ");
        else
            printf("\n ");
    }
}

int main(void)
{
    int year;
    int month;
    int first_day;

    do{
        printf("Enter a month (1 - 12): ");
        scanf("%d", &month);
    } while (month < 1 || month > 12);

    do{
        printf("Enter a year (1978 - 3000): ");
        scanf("%d", &year);
    } while (year < 1978 || year > 3000);

    first_day = first_day_year(year);
    leapyear(year);
    calendar(month, year, first_day);
   
    printf("\n");

    return 0;
}

---

Hm, I don't know. There are formulas to get from a Julian day number JDN to a Gregorian date, it's not Jan 1 1978 though. Ditto for converting UNIX epoch, which is seconds since Jan 1 1970.

Hmm.
The only problem I have now is where the first day of the month starts.
I really don't see what I can do to fix this.
Help, anyone? D: