#include <stdio.h>
#include <time.h>
/**
* Adds a specified number of weeks to a given struct tm date.
*
* @param date Pointer to struct tm representing the base date.
* @param weeks Number of weeks to add (can also be negative to subtract).
* @return 0 on success, -1 on failure (mktime error).
*/
int add_weeks_to_date(struct tm *date, int weeks) {
if (date == NULL) {
return -1;
}
// 1. Add the equivalent number of days (weeks * 7) to the day of the month.
// mktime() will handle overflow automatically (e.g., day 35 of a month).
date->tm_mday += weeks * 7;
// 2. Set tm_isdst to 0 to prevent Daylight Saving Time (DST) shifts
// from altering the calendar date during normalization.
date->tm_isdst = 0;
// 3. mktime normalizes the struct tm fields (recalculating year, month,
// day of week, and day of year) and converts it back to a time_t.
time_t normalized_time = mktime(date);
if (normalized_time == (time_t)(-1)) {
return -1; // Error during normalization
}
return 0;
}
int main(void) {
// Initialize a sample date: November 8, 2026
struct tm target_date = {0};
target_date.tm_year = 2026 - 1900; // Years since 1900
target_date.tm_mon = 10; // Month (0-11, so 10 = November)
target_date.tm_mday = 8; // Day of the month (1-31)
target_date.tm_hour = 12; // Noon, safely away from midnight DST shifts
int weeks_to_add = 4;
printf("Original date: %04d-%02d-%02d\n",
target_date.tm_year + 1900,
target_date.tm_mon + 1,
target_date.tm_mday);
if (add_weeks_to_date(&target_date, weeks_to_add) == 0) {
printf("After adding %d weeks: %04d-%02d-%02d\n",
weeks_to_add,
target_date.tm_year + 1900,
target_date.tm_mon + 1,
target_date.tm_mday);
} else {
fprintf(stderr, "Failed to compute new date.\n");
}
return 0;
}
/*
run:
Original date: 2026-11-08
After adding 4 weeks: 2026-12-06
*/
