CTIME(3)

NAME
asctime, ctime, gmtime, localtime, mktime - transform binary date and time to ASCII

SYNOPSIS
#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;

DESCRIPTION
The ctime(), gmtime() and localtime() functions all take an argument of data type time_t which represents calendar time. When interpreted as an absolute time value, it represents the number of seconds elapsed since 00:00:00 on January 1, 1970, Coordinated Universal Time (UTC).

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:

tm_sec
    The number of seconds after the minute, normally in the range 0 to 59, but can be up to 61 to allow for leap seconds.

tm_min
    The number of minutes after the hour, in the range 0 to 59.

tm_hour
    The number of hours past midnight, in the range 0 to 23.

tm_mday
    The day of the month, in the range 1 to 31.

tm_mon
    The number of months since January, in the range 0 to 11.

tm_year
    The number of years since 1900.

tm_wday
    The number of days since Sunday, in the range 0 to 6.

tm_yday
    The number of days since January 1, in the range 0 to 365.

tm_isdst
    A flag that indicates whether daylight saving time is in effect at the time described. The value is positive if daylight saving time is in effect, zero if it is not, and negative if the information is not available.

The ctime() function converts the calendar time timep into a string of the form

    "Wed Jun 30 21:49:08 1993\n"

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, expressed as local time, 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. If structure members are outside their legal interval, they will be normalized (so that, e.g., 40 October is changed into 9 November). 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 (seconds since the epoch), 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.

CONFORMING TO
SVID 3, POSIX, BSD 4.3, ISO 9899

SEE ALSO
date(1), gettimeofday(2), time(2), tzset(3), difftime(3), strftime(3).