time.h

time.h 定義了兩個宏：
NULL
CLOCKS_PER_SEC
包含
四個類型：
size_t
clock_t
time_t
struct tm

和九個函數：
clock_t clock(void);
double difftime(time_t time1, time_t time0);
time_t mktime(struct tm *timeptr);
time_t time(time_t *timer);

char *asctime(const struct tm *timeptr);
char *ctime(const time_t *timer);
struct tm *gmtime(const time_t *timer);
struct tm *localtime(const time_t *timer);
size_t strftime(char * restrict s, size_t maxsize, const char * restrict format, const struct tm * restrict timeptr);