#include <time.h>
size_t strftime(char *s,
                size_t slen,
                const char *fmt,
                const struct tm *tp); 

#include <wchar.h>
size_t wcsftime(wchar_t *s,
                size_t slen,
                const wchar_t *fmt,
                const struct tm *tp); 

The functions strftime and wcsftime take a null-terminated string fmt, and write it to an array of slen bytes or wide characters at s, converting the following sequences to textual representations of the fields in *tp:

The output is affected by the locale category LC_TIME. The functions return the number of bytes or characters written to the array at s, excluding the terminating null character. If the array is too small, the functions return zero.

#include <time.h>
char *asctime(const struct tm *tp);

#define __STDC_WANT_LIB_EXT1__ 1
#include <time.h>
errno_t asctime_s(char *buf,
                  rsize_t buflen,
                  const struct tm *tp);

The functions asctime and asctime_s convert a structural time *tp into a null-terminated string of the form Sun Sep 16 01:02:33 1973\n. asctime stores this in an internal buffer, and returns a pointer to its start, while asctime_s writes the string into the array of buflen bytes starting at buf. It returns zero on success, and non-zero on failure (also setting buf[0] to '\0' if buflen is at least 1).

#include <time.h>
char *ctime(const time_t *tp);

#define __STDC_WANT_LIB_EXT1__ 1
#include <time.h>
errno_t ctime_s(char *buf,
                rsize_t buflen,
                const time_t *tp);

The function ctime is equivalent to:

asctime(localtime(*tp));

The function ctime_s is equivalent to:

struct tm tmp;
asctime_s(buf, buflen, localtime_s(*tp, &tmp));

There is no standard function to convert from a textual time to a struct tm. However, a function called strptime exists on many systems for this purpose.