NEWCTIME(3)

NAME
asctime, ctime, difftime, gmtime, localtime, mktime - convert date and time to ASCII

SYNOPSIS
extern char *tzname[2];

void tzset()

#include <sys/types.h>

char *ctime(clock)
const time_t *clock;

double difftime(time1, time0)
time_t time1;
time_t time0;

#include <time.h>

char *asctime(tm)
const struct tm *tm;

struct tm *localtime(clock)
const time_t *clock;

struct tm *gmtime(clock)
const time_t *clock;

time_t mktime(tm)
struct tm *tm;

cc ... -ltz

DESCRIPTION
Ctime converts a long integer, pointed to by clock, representing the time in seconds since 00:00:00 UTC, 1970-01-01, and returns a pointer to a string of the form
Thu Nov 24 18:22:48 19860
Years requiring fewer than four characters are padded with leading zeroes. For years longer than four characters, the string is of the form
Thu Nov 24 18:22:48 819860
with five spaces before the year. These unusual formats are designed to make it less likely that older software that expects exactly 26 bytes of output will mistakenly output misleading values for out-of-range years.

Localtime and gmtime return pointers to ``tm'' structures, described below. Localtime corrects for the time zone and any time zone adjustments (such as Daylight Saving Time in the United States). After filling in the ``tm'' structure, localtime sets the tm_isdst'th element of tzname to a pointer to an ASCII string that's the time zone abbreviation to be used with localtime's return value.

Gmtime converts to Coordinated Universal Time.

Asctime converts a time value contained in a ``tm'' structure to a string, as shown in the above example, and returns a pointer to the string.

Mktime converts the broken-down time, expressed as local time, in the structure pointed to by tm into a calendar time value with the same encoding as that of the values returned by the time function. The original values of the tm_wday and tm_yday components of the structure are ignored, and the original values of the other components are not restricted to their normal ranges. (A positive or zero value for tm_isdst causes mktime to presume initially that summer time (for example, Daylight Saving Time in the U.S.A.) respectively, is or is not in effect for the specified time. A negative value for tm_isdst causes the mktime function to attempt to divine whether summer time is in effect for the specified time; in this case it does not use a consistent rule and may give a different answer when later presented with the same argument.) On successful completion, the values of the tm_wday and tm_yday components of the structure are set appropriately, and the other components are set to represent the specified calendar time, but with their values forced to their normal ranges; the final value of tm_mday is not set until tm_mon and tm_year are determined. Mktime returns the specified calendar time; If the calendar time cannot be represented, it returns -1.

Difftime returns the difference between two calendar times, (time1 - time0), expressed in seconds.

Declarations of all the functions and externals, and the ``tm'' structure, are in the <time.h> header file. The structure (of type) struct tm includes the following fields:

    	int tm_sec;	/* seconds (0 - 60) */
    	int tm_min;	/* minutes (0 - 59) */
    	int tm_hour;	/* hours (0 - 23) */
    	int tm_mday;	/* day of month (1 - 31) */
    	int tm_mon;	/* month of year (0 - 11) */
    	int tm_year;	/* year - 1900 */
    	int tm_wday;	/* day of week (Sunday = 0) */
    	int tm_yday;	/* day of year (0 - 365) */
    	int tm_isdst;	/* is summer time in effect? */
    	char *tm_zone;	/* abbreviation of timezone name */
    	long tm_gmtoff;	/* offset from UTC in seconds */

The tm_zone and tm_gmtoff fields exist, and are filled in, only if arrangements to do so were made when the library containing these functions was created. There is no guarantee that these fields will continue to exist in this form in future releases of this code.

Tm_isdst is non-zero if summer time is in effect.

Tm_gmtoff is the offset (in seconds) of the time represented from UTC, with positive values indicating east of the Prime Meridian.

FILES
/usr/share/zoneinfo    time zone information directory
/usr/share/zoneinfo/localtime    local time zone file
/usr/share/zoneinfo/posixrules    used with POSIX-style TZ's
/usr/share/zoneinfo/GMT    for UTC leap seconds

If /usr/share/zoneinfo/GMT is absent, UTC leap seconds are loaded from /usr/share/zoneinfo/posixrules.

SEE ALSO
getenv(3), newstrftime(3), newtzset(3), time(2), tzfile(5)

NOTES
The return values point to static data; the data is overwritten by each call. The tm_zone field of a returned struct tm points to a static array of characters, which will also be overwritten at the next call (and by calls to tzset).

Asctime and ctime behave strangely for years before 1000 or after 9999. The 1989 and 1999 editions of the C Standard say that years from -99 through 999 are converted without extra spaces, but this conflicts with longstanding tradition and with this implementation. Traditional implementations of these two functions are restricted to years in the range 1900 through 2099. To avoid this portability mess, new programs should use strftime instead.

Avoid using out-of-range values with mktime when setting up lunch with promptness sticklers in Riyadh.