#include <time.h> char *asctime(const struct tm *timeptr); char *ctime(const time_t *timep); struct tm *gmtime(const time_t *timep); struct tm *localtime(const time_t *timep); time_t mktime(struct tm *timeptr); extern char *tzname[2]; long int timezone; extern int daylight;
The asctime() and mktime() functions both take an argument representing broken-down time which is a binary representation separated into year, month, day, etc. Broken-down time is stored in the structure tm which is defined in <time.h> as follows:
struct tm { int tm_sec; /* seconds */ int tm_min; /* minutes */ int tm_hour; /* hours */ int tm_mday; /* day of the month */ int tm_mon; /* month */ int tm_year; /* year */ int tm_wday; /* day of the week */ int tm_yday; /* day in the year */ int tm_isdst; /* daylight saving time */ };
The members of the tm structure are:
The ctime() function converts the calendar time timep into a string of the form
The abbreviations for the days of the week are `Sun', `Mon', `Tue', `Wed', `Thu', `Fri', and `Sat'. The abbreviations for the months are `Jan', `Feb', `Mar', `Apr', `May', `Jun', `Jul', `Aug', `Sep', `Oct', `Nov', and `Dec'. The return value points to a statically allocated string which might be overwritten by subsequent calls to any of the date and time functions. The function also sets the external variable tzname with information about the current time zone.
The gmtime() function converts the calendar time timep to broken-down time representation, expressed in Coordinated Universal Time (UTC).
The localtime() function converts the calendar time timep to broken-time representation, expressed relative to the user's specified time zone. The function sets the external variables tzname with information about the current time zone, timezone with the difference between Coordinated Universal Time (UTC) and local standard time in seconds, and daylight to a non-zero value if standard US daylight savings time rules apply.
The asctime() function converts the broken-down time value timeptr into a string with the same format as ctime(). The return value points to a statically allocated string which might be overwritten by subsequent calls to any of the date and time functions.
The mktime() function converts a broken-down time structure to calendar time representation. The function ignores the specified contents of the structure members tm_wday and tm_yday and recomputes them from the other information in the broken-down time structure. Calling mktime() also sets the external variable tzname with information about the current time zone. If the specified broken-down time cannot be represented as calendar time, mktime() returns a value of (time_t)(-1) and does not alter the tm_wday and tm_yday members of the broken-down time structure.