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DESCRIPTION

Years requiring fewer than four characters are padded with leading zeroes. For years longer than four characters, the string is of the form

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.

The ctime_r() function converts the calendar time pointed to by clock to local time in exactly the same way as ctime() and puts the string into the array pointed to by buf (which contains at least 26 bytes) and returns buf. Unlike ctime(), the thread-safe version ctime_r() is not required to set tzname.

The localtime() and gmtime() functions 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 the return value of localtime().

gmtime() converts to Coordinated Universal Time.

The localtime_r() and gmtime_r() functions convert the calendar time pointed to by clock into a broken-down time in exactly the same way as their non-reentrant counterparts, localtime() and gmtime(), but instead store the result directly into the structure pointed to by result. Unlike localtime(), the reentrant version is not required to set tzname.

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. asctime_r() uses the buffer pointed to by buf (which should contain at least 26 bytes) and then returns buf.

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.

timelocal() is a deprecated interface that is equivalent to calling mktime() with a negative value for tm_isdst.

timegm() converts the broken-down time, as returned by gmtime(), into a calendar time value with the same encoding as that of the values returned by the time() function.

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? */
	long tm_gmtoff;	/* offset from UTC in seconds */
	char *tm_zone;	/* abbreviation of timezone name */

The tm_zone and tm_gmtoff fields exist, and are filled in by mktime(), localtime(), timegm(), and gmtime(), but are not standardized. There is no guarantee that these fields will continue to exist in this form and they may be altered or removed in a future release.

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.