newctime(3) Library Functions Manual newctime(3)

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

#include <time.h>
extern char *tzname[]; /∗ (optional) ∗/
[[deprecated]] char *ctime(time_t const *clock);
char *ctime_r(time_t const *clock, char *buf);
double difftime(time_t time1, time_t time0);
[[deprecated]] char *asctime(struct tm const *tm);
char *asctime_r(struct tm const *restrict tm,
    char *restrict result);
struct tm *localtime(time_t const *clock);
struct tm *localtime_r(time_t const *restrict clock,
    struct tm *restrict result);
struct tm *localtime_rz(timezone_t restrict zone,
    time_t const *restrict clock,
    struct tm *restrict result);
struct tm *gmtime(time_t const *clock);
struct tm *gmtime_r(time_t const *restrict clock,
    struct tm *restrict result);
time_t mktime(struct tm *tm);
time_t mktime_z(timezone_t restrict zone,
    struct tm *restrict tm);
cc ... -ltz

The ctime function converts a long integer, pointed to by clock, and returns a pointer to a string of the form

Thu Nov 24 18:22:48 1986\n\0

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 81986\n\0

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 *clock timestamp represents the time in seconds since 1970-01-01 00:00:00 Coordinated Universal Time (UTC). The POSIX standard says that timestamps must be nonnegative and must ignore leap seconds. Many implementations extend POSIX by allowing negative timestamps, and can therefore represent timestamps that predate the introduction of UTC and are some other flavor of Universal Time (UT). Some implementations support leap seconds, in contradiction to POSIX.

The ctime function is deprecated starting in C23. Callers can use localtime_r and strftime instead.

The localtime and gmtime functions return pointers to “tm” structures, described below. The localtime function 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 a string that's the time zone abbreviation to be used with localtime's return value.

The gmtime function converts to Coordinated Universal Time.

The asctime function 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. This function is deprecated starting in C23. Callers can use strftime instead.

The mktime function 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 daylight saving time 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 daylight saving 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. The mktime function returns the specified calendar time; If the calendar time cannot be represented, it returns -1.

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

The ctime_r, localtime_r, gmtime_r, and asctime_r functions are like their unsuffixed counterparts, except that they accept an additional argument specifying where to store the result if successful.

The localtime_rz and mktime_z functions are like their unsuffixed counterparts, except that they accept an extra initial zone argument specifying the timezone to be used for conversion. If zone is null, UT is used; otherwise, zone should be have been allocated by tzalloc and should not be freed until after all uses (e.g., by calls to strftime) of the filled-in tm_zone fields.

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 daylight saving time in effect? ∗/
char ∗tm_zone;	/∗ time zone abbreviation (optional) ∗/
long tm_gmtoff;	/∗ offset from UT in seconds (optional) ∗/

The tm_isdst field is non-zero if daylight saving time is in effect.

The tm_gmtoff field is the offset (in seconds) of the time represented from UT, with positive values indicating east of the Prime Meridian. The field's name is derived from Greenwich Mean Time, a precursor of UT.

In struct tm 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. Similarly, the tzname variable is optional; also, there is no guarantee that tzname will continue to exist in this form in future releases of this code.

/etc/localtime local timezone file
/usr/share/zoneinfo timezone directory
/usr/share/zoneinfo/posixrules default DST rules (obsolete)
/usr/share/zoneinfo/GMT for UTC leap seconds

If /usr/share/zoneinfo/GMT is absent, UTC leap seconds are loaded from /usr/share/zoneinfo/GMT0 if present.

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

The return values of asctime, ctime, gmtime, and localtime point to static data overwritten by each call. The tzname variable (once set) and the tm_zone field of a returned struct tm both point to an array of characters that can be freed or overwritten by later calls to the functions localtime, tzfree, and tzset, if these functions affect the timezone information that specifies the abbreviation in question. The remaining functions and data are thread-safe.

The asctime, asctime_r, ctime, and ctime_r functions 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. The 2011 edition says that the behavior is undefined if the year is before 1000 or after 9999. 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.

Time Zone Database