Algorithm for the calender?

Question

Suppose given a date how can we find out the day i.e. sunday, monday .....etc.

Please help me, google is confusing me.

Answer 1

#include <stdio.h>
#include <time.h>   // For mktime(),  strftime and struct tm
#include <string.h> // For memset()

int main()
{
    int month;
    int day;
    int year;
    char daybuf[20];
    struct tm time_str;

    printf("Enter the year (>= 1900) : ");
    scanf("%d", &year);
    printf("Enter the month number (1 -12) : ");
    scanf("%d", &month);
    printf("Enter the day number (1 - 31) : ");
    scanf("%d", &day);

    printf("Your enterd date is (dd/mm/yy) : %d/%d/%d\n", day, month, year);

    memset(&time_str, 0, sizeof(time_str));

    time_str.tm_year = year - 1900;     // Year since 1900
    time_str.tm_mon = month - 1;        // Month of year [0, 11]
    time_str.tm_mday = day;
    time_str.tm_isdst = -1;
    /*
      The value of tm_isdst shall be positive if Daylight Savings Time is in effect, 0 if Daylight Savings Time 
       is not in effect, and negative if the information is not available.
       For more info please look through the <time.h> file.
    */


    if (mktime(&time_str) == -1)
        printf("-Unknown-");
    else {
        strftime(daybuf, sizeof(daybuf), "%A", &time_str);
        /*
            For more information how to get different contents of "structure tm" 
            do a "man 3 strftime"
        */
        printf("\nOutput Day : %s\n", daybuf);
    }
    return 0;
}