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CTIME(3)                 BSD 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, tzalloc,
     tzgetname, tzfree, -- convert date and time to ASCII

     Standard C Library (libc, -lc)

     #include <time.h>

     extern char *tzname[2];

     char *
     asctime(const struct tm *tm);

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

     char *
     ctime(const time_t *clock);

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

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

     mktime(struct tm *tm);

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

     tzalloc(const char *zone);

     tzfree(const timezone_t tz);

     const char *
     tzgetname(const timezone_t tz, int isdst);

     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).

     asctime_r(tm, buf)
           The asctime_r() has the same behavior as asctime(), but the result
           is stored to 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.

     ctime_r(clock, buf)
           The ctime_r() is similar to ctime(), except it places the result of
           the conversion on 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 const timezone_t argument, as returned by a previous call to
           tzalloc(), or a null pointer denoting Coordinated Universal Time

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

           Also localtime() is comparable to gmtime().  However, localtime()
           corrects for the time zone and any time zone 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 time zone 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 to 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 const 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.

           +o   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.

           +o   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.

           +o   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

           The tzalloc() function takes as an argument a timezone name and
           returns a timezone_t object suitable to be used in the ctime_rz(),
           localtime_rz(), and mktime_z() functions.

           A null pointer may be passed to tzalloc() instead of a timezone
           name, to refer to Coordinated Universal Time (UTC).

           Note that instead of setting the environment variable TZ, and
           globally changing the behavior of the calling program, one can use
           multiple timezones at the same time by using separate timezone_t
           objects allocated by tzalloc() and calling the "z" variants of the

           The tzfree() function deallocates tz, which was previously
           allocated by tzalloc().

           Finally, tzgetname() returns the name for the given tz.  If isdst
           is 0, the call is equivalent to tzname[0].  If isdst is set to 1
           the call is equivalent to tzname[1].

     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 UT 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.

     +o   tm_isdst is non-zero if summer time is in effect.

     +o   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.

     +o   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.

     +o   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.

     +o   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.

     +o   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).

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

     /etc/localtime                  local time zone file
     /usr/share/zoneinfo             time zone information directory
     /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

     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").

     The functions that do not take an explicit timezone_t argument return
     values point to static data; the data is overwritten by each call.  For
     the above functions 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(3)).  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() and ctime() 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.
     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.

BSD                           September 20, 2013                           BSD