My C program that uses Zeller's algorithm is very buggy on leap years, not sure why?

Question

The program is supposed to return what day of the week it is for the entered date. One of the dates that doesn't work is 01012000. Nothing is returned at all. But on some other leap years the first day of March can be calculated. Also sometimes seemingly random dates don't work. I'm not sure how to fix this. Also I'm supposed to write the part that calculates "daynumber" and then call on it later so I'm not sure if I'm doing that right.

Sorry for the beginner questions, this is my first ever C program.

#include<stdio.h>
#include<math.h>
int main()
{
int day, month, year, lastday, dayname, daynumber, input, d;

//Determine the last day of user specified month

printf("Enter date: ddmmyyyy:\n");
scanf("%d", &input);

day = input/1000000;
month = (input/10000) % 100;
year = input % 10000;

if (month == 1 || month == 3 || month == 5 || month == 7 || 
    month == 8 || month == 10 || month == 12)
    lastday = 31;
else if (month == 4 || month == 6 || month == 9 || month == 11)
    lastday = 30;
else if ((year%4 == 0 && year%100 !=0) || year%400 == 0)
    lastday = 29;
else
    lastday = 28;

//Verify the date

if (year < 0)
    return 1;
if (month < 1 || month > 12)
    return 2;
if (day < 1 || day > lastday)
    return 3;

//Algorithm

{

int m, d, y, c, daynumber;

if (month > 3)
    m = month - 2;
else
    m = month + 10;

if (m == 11 || m == 12)
    year = year - 1;
else
    year = year;

d = day;
y = year % 100;
c = year / 100;

daynumber = (((13*m - 1)/5) + d + y + (y/4) + (c/4) - 2*c) % 7;

if (daynumber == 0)
    printf("Sunday\n");
if (daynumber == 1)
    printf("Monday\n");
if (daynumber == 2)
    printf("Tuesday\n");
if (daynumber == 3)
    printf("Wednesday\n");
if (daynumber == 4)
    printf("Thursday\n");
if (daynumber == 5)
    printf("Friday\n");
if (daynumber == 6)
    printf("Saturday\n");

}

}

Use the `ctime` library to manage dates. Daylight savings, leap days and leap seconds complicate writing your own algorithm if you care about accuracy. — IceArdor, Feb 13 '14 at 08:04
@IceArdor: Zeller's congruence already accounts for leap years. Since this is a beginner's question, it might be preferable to help them rather than teach them how to stitch libraries together. ;) — Michael Foukarakis, Feb 13 '14 at 08:07
Aren't months in Zeller's congruence counted from 3 = March to 14 = February? You shouldn't be subtracting from `month`, only adding `+12` for January & February, and consequently only decrementing the year when `m == 13 || m == 14`, right? — Michael Foukarakis, Feb 13 '14 at 08:12

score 1 · Answer 1 · edited May 23 '17 at 11:57

I can't see your bug, but there is no need to scan the whole number and divide, use:

scanf("%2d%2d%4d", &day, &month, &year);

Using Sakamoto's algorithm you can do the same in few lines:

#include <stdio.h>

static int wday(int d, int m, int y)
{
    static int offset[] = {0, 3, 2, 5, 0, 3, 5, 1, 4, 6, 2, 4};

    y -= m < 3;
    return (y + y / 4 - y / 100 + y / 400 + offset[m - 1] + d) % 7;
}

int main(void)
{
    const char *days[] = {"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"};
    int d, m, y;

    printf("Enter date [ddmmyyyy]: ");
    scanf("%2d%2d%4d", &d, &m, &y);
    printf("%s\n", days[wday(d, m, y)]);
    return 0;
}

score 1 · Answer 2 · answered Feb 13 '14 at 07:58

This

(((13*m - 1)/5) + d + y + (y/4) + (c/4) - 2*c)

can probably be negative. The result of % 7 would then still be negative and nothing is printed.

Just add daynumber = (daynumber + 7) % 7; after the line

daynumber = (((13*m - 1)/5) + d + y + (y/4) + (c/4) - 2*c) % 7;

score 0 · Answer 3 · answered Feb 13 '14 at 06:55

You are using:

 printf("Enter date: ddmmyyyy:\n");
 scanf("%d", &input);

Here you are storing the input as a int. If sizeof(int) is 2 byte, its range would be between -32,768 to 32,767 and your input would be out of the range. For this you should use long int.

My C program that uses Zeller's algorithm is very buggy on leap years, not sure why?

3 Answers3