Question

How to add N weeks to the current date in C

Answer 1

#include <stdio.h>
#include <time.h>

/**
 * Adds a specified number of weeks to the current date.
 *
 * @param weeks       The number of weeks to add (supports negative numbers to subtract).
 * @param out_buffer  Buffer to store the resulting formatted date string (e.g., "YYYY-MM-DD").
 * @param buffer_size The size of the output buffer.
 * @return int        Returns 0 on success, or -1 on failure.
 */
int addWeeksToCurrentDate(int weeks, char *out_buffer, size_t buffer_size) 
{
    // 1. Get the current calendar time
    time_t current_time = time(NULL);
    if (current_time == (time_t)-1) {
        return -1; // Failed to retrieve system time
    }

    // 2. Convert raw time to a broken-down local time structure
    struct tm *time_info = localtime(&current_time);
    if (time_info == NULL) {
        return -1; // Failed to convert time
    }

    // 3. Add the weeks by adjusting the day of the month (1 week = 7 days)
    // Note: Modifying tm_mday directly and letting mktime normalize is idiomatic in C.
    time_info->tm_mday += weeks * 7;

    // 4. Normalize the struct tm using mktime()
    // mktime automatically handles overflows (e.g., if day exceeds month length)
    time_t updated_time = mktime(time_info);
    if (updated_time == (time_t)-1) {
        return -1; // Failed to normalize time
    }

    // 5. Format the resulting date into an ISO-8601 string (YYYY-MM-DD)
    if (strftime(out_buffer, buffer_size, "%Y-%m-%d", time_info) == 0) {
        return -1; // Buffer too small or formatting error
    }

    return 0;
}

int main() 
{
    int weeks_to_add = 4;
    char date_buffer[11]; // Space for "YYYY-MM-DD" + null terminator

    if (addWeeksToCurrentDate(weeks_to_add, date_buffer, sizeof(date_buffer)) == 0) {
        printf("Date after adding %d weeks: %s\n", weeks_to_add, date_buffer);
    } else {
        printf("An error occurred while calculating the date.\n");
    }

    return 0;
}


/*
run:

Date after adding 4 weeks: 2026-12-05

*/

 

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Answer 2

#include <stdio.h>
#include <time.h>

/**
 * Adds a specified number of weeks to the current date.
 *
 * @param weeks       The number of weeks to add (supports negative numbers to subtract).
 * @param out_buffer  Buffer to store the resulting formatted date string (e.g., "YYYY-MM-DD").
 * @param buffer_size The size of the output buffer.
 * @return int        Returns 0 on success, or -1 on failure.
 */
int addWeeksToCurrentDate(int weeks, char *out_buffer, size_t buffer_size) 
{
    // 1. Get the current calendar time
    time_t current_time = time(NULL);
    if (current_time == (time_t)-1) {
        return -1; // Failed to retrieve system time
    }

    // 1 week = 7 days * 24 hours * 60 minutes * 60 seconds = 604800 seconds
    time_t future_time = current_time + ((time_t)weeks * 7 * 24 * 60 * 60);

    struct tm *time_info = localtime(&future_time);
    if (time_info == NULL) {
        return -1; // Failed to convert time
    }
    
    // 5. Format the resulting date into an ISO-8601 string (YYYY-MM-DD)
    if (strftime(out_buffer, buffer_size, "%Y-%m-%d", time_info) == 0) {
        return -1; // Buffer too small or formatting error
    }

    return 0;
}

int main() 
{
    int weeks_to_add = 4;
    char date_buffer[11]; // Space for "YYYY-MM-DD" + null terminator

    if (addWeeksToCurrentDate(weeks_to_add, date_buffer, sizeof(date_buffer)) == 0) {
        printf("Date after adding %d weeks: %s\n", weeks_to_add, date_buffer);
    } else {
        printf("An error occurred while calculating the date.\n");
    }

    return 0;
}


/*
run:

Date after adding 4 weeks: 2026-12-05

*/

 

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