apue_gmtime.h

#include <time.h>

struct tm *gmtime(const time_t *calptr);
struct tm *localtime(const time_t *calptr);

/* Both return: pointer to broken-down time, NULL on error */

apue_mktime.h

#include <time.h>

time_t mktime(struct tm *tmptr);

Returns: calendar time if OK, −1 on error

apue_strftime.h

#include <time.h>

size_t strftime(char *restrict buf, size_t maxsize,
                const char *restrict format,
                const struct tm *restrict tmptr);

size_t strftime_l(char *restrict buf, size_t maxsize,
                  const char *restrict format,
                  const struct tm *restrict tmptr, locale_t locale);

/* Both return: number of characters stored in array if room, 0 otherwise */

apue_strptime.h

#include <time.h>

char *strptime(const char *restrict buf, const char *restrict format,
               struct tm *restrict tmptr);
               
/* Returns: pointer to one character past last character parsed, NULL otherwise */