localtime() / gmtime() / mktime()

// Converts a time_t value to a local-time struct tm (not thread-safe).
// Returns a pointer to an internal static struct tm (overwritten on the next call).
struct tm *localtime(const time_t *timer);

// Converts a time_t value to a UTC struct tm (not thread-safe).
// Returns a pointer to an internal static struct tm.
struct tm *gmtime(const time_t *timer);

// Converts a struct tm back to a time_t value (the inverse of localtime).
// Returns the time_t value on success, or (time_t)(-1) on error.
time_t mktime(struct tm *tm);

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

int main(void) {
    // Get the current time.
    time_t now = time(NULL);

    // Convert to local time using localtime.
    struct tm *local = localtime(&now);
    printf("Local time:\n");
    printf("  %04d-%02d-%02d %02d:%02d:%02d\n",
        local->tm_year + 1900, // tm_year is years since 1900, so add 1900.
        local->tm_mon + 1,     // tm_mon is zero-based, so add 1.
        local->tm_mday,
        local->tm_hour,
        local->tm_min,
        local->tm_sec);

    // Display the day of the week as a string.
    const char *weekdays[] = {"Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"};
    printf("  Day of week: %s\n", weekdays[local->tm_wday]);
    printf("  Day of year: %d\n", local->tm_yday + 1);

    // Get UTC time using gmtime (9 hours behind Japan Standard Time).
    struct tm *utc = gmtime(&now);
    printf("UTC time: %02d:%02d:%02d\n", utc->tm_hour, utc->tm_min, utc->tm_sec);

    // Convert a specific date and time to time_t using mktime.
    struct tm target = {0}; // Zero-initialize all members.
    target.tm_year = 2024 - 1900; // Year 2024
    target.tm_mon  = 0;           // January (zero-based)
    target.tm_mday = 1;           // 1st
    target.tm_hour = 0;
    target.tm_min  = 0;
    target.tm_sec  = 0;
    time_t new_year = mktime(&target);
    printf("Timestamp for Jan 1, 2024: %ld\n", (long)new_year);

    return 0;
}

A common source of bugs with struct tm is that tm_year stores the year minus 1900, and tm_mon is zero-based. To display the correct year, use tm_year + 1900; for the correct month, use tm_mon + 1.

localtime() and gmtime() are not thread-safe. They return a pointer to an internal static buffer, so calling them simultaneously from multiple threads causes a data race. In multithreaded environments, use the POSIX extensions localtime_r() / gmtime_r() (or localtime_s() on Windows) instead.

mktime() automatically normalizes out-of-range values (for example, tm_mday = 32). January 32nd is corrected to February 1st. You can use this behavior to perform date arithmetic. In addition, mktime() automatically calculates and fills in tm_wday (day of the week) and tm_yday (day of the year).

To convert a time value to a string, use strftime() / asctime() / ctime(). For obtaining a time_t value, see time() / difftime() / clock().