Updated: 2022/Sep/29

Please read Privacy Policy. It's for your privacy.

CTIME(3)                   Library Functions Manual                   CTIME(3)

     asctime, asctime_r, ctime, ctime_r, ctime_rz, difftime, gmtime, gmtime_r,
     localtime, localtime_r, localtime_rz, mktime, mktime_z - convert date and

     Standard C Library (libc, -lc)

     #include <time.h>

     extern char *tzname[2];

     [[deprecated]] char *
     asctime(const struct tm *tm);

     char *
     asctime_r(const struct tm *restrict tm, char * restrict buf);

     [[deprecated]] char *
     ctime(const time_t *clock);

     char *
     ctime_r(const time_t *clock, char *buf);

     char *
     ctime_rz(timezone_t restrict tz, const time_t *clock, char *buf);

     difftime(time_t time1, time_t time0);

     struct tm *
     gmtime(const time_t *clock);

     struct tm *
     gmtime_r(const time_t * restrict clock, struct tm * restrict result);

     struct tm *
     localtime(const time_t *clock);

     struct tm *
     localtime_r(const time_t * restrict clock, struct tm * restrict result);

     struct tm *
     localtime_rz(timezone_t restrict tz, const time_t * restrict clock,
         struct tm * restrict result);

     mktime(struct tm *tm);

     mktime_z(timezone_t restrict tz, struct tm *restrict tm);

     The asctime family of functions provide various standard library routines
     to operate with time and conversions related to time.

           The asctime() function converts a time value contained in the tm
           structure to a string with the following general format:

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

           The tm structure is described in tm(3).

           This function is deprecated starting in C23.  Callers can use
           strftime() instead.

     asctime_r(tm, buf)
           The asctime_r() has the same behavior as asctime(), but the result
           is stored in buf, which should have a size of at least 26 bytes.

           The ctime() function converts a time_t, pointed to by clock, and
           returns a pointer to a string with the format described above.
           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 time stamp represents the time in seconds since
           1970-01-01 00:00:00 Coordinated Universal Time (UTC).  The POSIX
           standard says that time stamps must be nonnegative and must ignore
           leap seconds.  Many implementations extend POSIX by allowing
           negative time stamps, and can therefore represent time stamps 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.

     ctime_r(clock, buf)
           The ctime_r() is similar to ctime(), except it places the result of
           the conversion in the buf argument, which should be 26 or more
           bytes long, instead of using a global static buffer.

     ctime_rz(tz, clock, buf)
           The ctime_rz() function is similar to ctime_r(), but it also takes
           a timezone_t argument, as returned by a previous call to tzalloc(),
           or a NULL pointer denoting Coordinated Universal Time (UTC).

     difftime(time1, time2)
           The difftime() function returns the difference between two calendar
           times, (time1 - time0), expressed in seconds.

           The gmtime() function converts to Coordinated Universal Time (UTC)
           and returns a pointer to the tm structure described in tm(3).

     gmtime_r(clock, result)
           The gmtime_r() function provides the same functionality as
           gmtime(), differing in that the caller must supply a buffer area
           result in which the result is stored.

           Also localtime() is comparable to gmtime().  However, localtime()
           corrects for the timezone and any timezone adjustments (such as
           Daylight Saving Time in the U.S.A.).  After filling in the tm
           structure, the function sets the tm_isdst'th element of tzname to a
           pointer to an ASCII string that is the timezone abbreviation to be
           used with localtime()'s return value.

     localtime_r(clock, result)
           As gmtime_r(), the localtime_r() takes an additional buffer result
           as a parameter and stores the result in it.  Note however that
           localtime_r() does not imply initialization of the local time
           conversion information; the application may need to do so by
           calling tzset(3).

     localtime_rz(tz, clock, result)
           The localtime_rz() function is similar to localtime_r(), but it
           also takes a timezone_t argument, returned by a previous call to
           tzalloc(), or a NULL pointer denoting Coordinated Universal Time

           The mktime() function converts the broken-down time, expressed as
           local time in the tm(3) structure, into a calendar time value with
           the same encoding as that of the values returned by the time(3)
           function.  The following remarks should be taken into account.

              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 and will
               be normalized, if need be.

               For example, consider a struct tm initialized with tm_year =
               122, tm_mon = 10, tm_mday = 30, tm_hour = 22, tm_min = 57, and
               tm_sec = 0.  Incrementing tm_min by 13 and calling mktime()
               would lead to tm_hour = 23 and tm_min = 10.

               This normalizing can lead to cascading changes: Again using a
               struct tm initialized as in the above example but with tm_hour
               = 23, the same change would lead to tm_mon = 11, tm_mday = 1,
               tm_hour = 0, and tm_min = 10.

               Negative values may also be normalized with similar cascading
               effect such that e.g., a tm_hour of -1 means 1 hour before
               midnight on the previous day and so on.

              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

           The function returns the specified calendar time; if the calendar
           time cannot be represented, it returns (time_t)-1.  This can happen
           either because the resulting conversion would not fit in a time_t
           variable, or because the time specified happens to be in the
           daylight savings gap and tm_isdst was set to -1.  Other mktime()
           implementations do not return an error in the second case and
           return the appropriate time offset after the daylight savings gap.
           There is code to mimick this behavior, but it is not enabled by

     mktime_z(tz, tm)
           The mktime_z() function is similar to mktime() but it also takes a
           const timezone_t argument, returned by a previous call to
           tzalloc(), or a null pointer denoting Coordinated Universal Time

     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;   /* abbreviation of timezone name (optional) */
            long tm_gmtoff;  /* offset from UT in seconds (optional) */

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

        tm_gmtoff 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.

     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 ctime_rz(), 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 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.

        On success the asctime() and ctime() functions return a pointer to a
         static character buffer, and the asctime_r(), ctime_r(), and
         ctime_rz() function return a pointer to the user-supplied buffer.  On
         failure they all return NULL and no errors are defined for them.

        On success the gmtime(), and localtime() functions return a pointer
         to a statically allocated struct tm whereas the gmtime_r(),
         localtime_r(), and localtime_rz(), functions return a pointer to the
         user-supplied struct tm.  On failure they all return NULL and the
         global variable errno is set to indicate the error.

        The mktime() and mktime_z() function returns the specified time since
         the Epoch as a time_t type value.  If the time cannot be represented,
         then mktime() and mktime_z() return (time_t)-1 setting the global
         variable errno to indicate the error.

        The tzalloc() function returns a pointer to a timezone_t object or
         NULL on failure, setting errno to indicate the error.  It may also
         return NULL when the name argument is NULL, and this is not an error,
         but a way of referring to Coordinated Universal Time (UTC).

        tzgetzone() function returns string containing the name of the
         timezone given in tz.

     /etc/localtime                  local timezone file
     /usr/share/zoneinfo             timezone information directory
     /usr/share/zoneinfo/localtime   local timezone file
     /usr/share/zoneinfo/posixrules  default DST rules (obsolete, and can
                                     cause bugs if present)
     /usr/share/zoneinfo/GMT         for UTC leap seconds

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

     The described functions may fail with

     [EINVAL]           The result cannot be represented because a parameter
                        is incorrect, or the conversion failed because no such
                        time exists (for example a time in the DST gap).

     [EOVERFLOW]        The result cannot be represented because the time
                        requested is out of bounds and the time calculation
                        resulted in overflow.

     All functions that return values, except their "z" variants, can also
     return the same errors as open(2) and malloc(3).

     getenv(3), strftime(3), time(3), tm(3), tzset(3), tzfile(5)

     The ctime(), difftime(), asctime(), localtime(), gmtime() and mktime()
     functions conform to ANSI X3.159-1989 ("ANSI C89").  Rest of the
     functions conform to IEEE Std 1003.1-2008 ("POSIX.1").

     A ctime() function appeared in Version 1 AT&T UNIX.

     The functions that do not take an explicit timezone_t argument return
     values pointing to static data; the data is overwritten by each call.
     For the above functions the tzname variable (once set) and the tm_zone
     field of a returned struct tm 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 functions that do take an explicit timezone_t
     argument and set the fields of a supplied struct tm should not call
     tzfree() since the tm_zone field of the struct tm points to data
     allocated by tzalloc().

     The asctime(), asctime_r(), ctime(), ctime_r(), and ctime_rz(), 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.

NetBSD 10.99                  September 16, 2023                  NetBSD 10.99