strftime() / asctime() / ctime()

// Converts a struct tm to a string according to a format string.
// Returns: the number of characters written (excluding the null terminator '\0'), or 0 if the buffer is too small.
size_t strftime(char *str, size_t maxsize, const char *format, const struct tm *tm);

// Converts a struct tm to a fixed-format English string (not thread-safe).
// Returns: a pointer to a static string in the format "Www Mmm dd hh:mm:ss yyyy\n".
char *asctime(const struct tm *tm);

// Converts a time_t to a fixed-format English string (not thread-safe).
// Returns: a pointer to a string equivalent to asctime(localtime(timer)).
char *ctime(const time_t *timer);

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

int main(void) {
    time_t now = time(NULL);
    struct tm *local = localtime(&now);

    // Use strftime to format the time in any format you like.
    char buf[128];

    // Output in ISO 8601 format (international standard, suitable for logs).
    strftime(buf, sizeof(buf), "%Y-%m-%dT%H:%M:%S", local);
    printf("ISO 8601: %s\n", buf); // e.g., "2024-03-15T10:30:05"

    // Format suitable for filenames (avoids spaces and colons).
    strftime(buf, sizeof(buf), "%Y%m%d_%H%M%S", local);
    printf("Filename: %s\n", buf); // e.g., "20240315_103005"

    // Use asctime to convert to a fixed English format.
    printf("asctime: %s", asctime(local)); // The string already ends with '\n'.
    // e.g., "Fri Mar 15 10:30:05 2024"

    // Use ctime to convert a time_t directly to a string (equivalent to localtime + asctime).
    printf("ctime:   %s", ctime(&now));
    // e.g., "Fri Mar 15 10:30:05 2024"

    return 0;
}

strftime() is the most flexible and practical of the three functions, as it lets you specify any format using format specifiers. For logging timestamps, the ISO 8601 format (%Y-%m-%dT%H:%M:%S) is widely used internationally.

%A (weekday name) and %B (month name) are locale-dependent and may output in languages other than English depending on the system locale. For portability, consider using a fixed array instead (e.g., weekdays[] = {"Sun", "Mon", ...}).

asctime() and ctime() are not thread-safe. They return a pointer to an internal static buffer, so calling them from multiple threads simultaneously can cause a data race. Also note that the string returned by asctime() already includes a trailing newline ('\n'), so avoid adding an extra newline when passing it to printf().

For information on obtaining a struct tm, see localtime() / gmtime() / mktime(). For obtaining a time_t, see time() / difftime() / clock().