gmtime(3)


NAME

   asctime,   ctime,   gmtime,   localtime,  mktime,  asctime_r,  ctime_r,
   gmtime_r, localtime_r - transform date and time to broken-down time  or
   ASCII

SYNOPSIS

   #include <time.h>

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

   char *ctime(const time_t *timep);
   char *ctime_r(const time_t *timep, char *buf);

   struct tm *gmtime(const time_t *timep);
   struct tm *gmtime_r(const time_t *timep, struct tm *result);

   struct tm *localtime(const time_t *timep);
   struct tm *localtime_r(const time_t *timep, struct tm *result);

   time_t mktime(struct tm *tm);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

   asctime_r(), ctime_r(), gmtime_r(), localtime_r():
          _POSIX_C_SOURCE
              || /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

DESCRIPTION

   The ctime(), gmtime() and localtime() functions all take an argument of
   data type time_t, which represents calendar time.  When interpreted  as
   an  absolute  time  value,  it represents the number of seconds elapsed
   since the Epoch, 1970-01-01 00:00:00 +0000 (UTC).

   The asctime() and mktime() functions both take an argument representing
   broken-down time, which is a representation separated into year, month,
   day, and so on.

   Broken-down time is stored in the structure tm,  which  is  defined  in
   <time.h> as follows:

       struct tm {
           int tm_sec;    /* Seconds (0-60) */
           int tm_min;    /* Minutes (0-59) */
           int tm_hour;   /* Hours (0-23) */
           int tm_mday;   /* Day of the month (1-31) */
           int tm_mon;    /* Month (0-11) */
           int tm_year;   /* Year - 1900 */
           int tm_wday;   /* Day of the week (0-6, Sunday = 0) */
           int tm_yday;   /* Day in the year (0-365, 1 Jan = 0) */
           int tm_isdst;  /* Daylight saving time */
       };

   The members of the tm structure are:

   tm_sec    The number of seconds after the minute, normally in the range
             0 to 59, but can be up to 60 to allow for leap seconds.

   tm_min    The number of minutes after the hour, in the range 0 to 59.

   tm_hour   The number of hours past midnight, in the range 0 to 23.

   tm_mday   The day of the month, in the range 1 to 31.

   tm_mon    The number of months since January, in the range 0 to 11.

   tm_year   The number of years since 1900.

   tm_wday   The number of days since Sunday, in the range 0 to 6.

   tm_yday   The number of days since January 1, in the range 0 to 365.

   tm_isdst  A flag that indicates whether  daylight  saving  time  is  in
             effect  at  the  time  described.   The  value is positive if
             daylight saving time is in effect, zero if  it  is  not,  and
             negative if the information is not available.

   The  call ctime(t) is equivalent to asctime(localtime(t)).  It converts
   the calendar time t into a null-terminated string of the form

          "Wed Jun 30 21:49:08 1993\n"

   The abbreviations for the days of the week  are  "Sun",  "Mon",  "Tue",
   "Wed",  "Thu",  "Fri", and "Sat".  The abbreviations for the months are
   "Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug",  "Sep",  "Oct",
   "Nov",  and  "Dec".   The return value points to a statically allocated
   string which might be overwritten by subsequent calls  to  any  of  the
   date and time functions.  The function also sets the external variables
   tzname, timezone, and daylight (see tzset(3))  with  information  about
   the  current  timezone.  The reentrant version ctime_r() does the same,
   but stores the string in a user-supplied buffer which should have  room
   for at least 26 bytes.  It need not set tzname, timezone, and daylight.

   The  gmtime()  function converts the calendar time timep to broken-down
   time representation, expressed in Coordinated Universal Time (UTC).  It
   may return NULL when the year does not fit into an integer.  The return
   value  points  to  a  statically  allocated  struct  which   might   be
   overwritten  by subsequent calls to any of the date and time functions.
   The gmtime_r() function does the same, but stores the data in  a  user-
   supplied struct.

   The  localtime()  function  converts the calendar time timep to broken-
   down time representation, expressed relative to  the  user's  specified
   timezone.   The  function  acts  as  if it called tzset(3) and sets the
   external variables tzname with information about the current  timezone,
   timezone  with  the difference between Coordinated Universal Time (UTC)
   and local standard time in seconds, and daylight to a nonzero value  if
   daylight  savings  time  rules apply during some part of the year.  The
   return value points to a statically allocated  struct  which  might  be
   overwritten  by subsequent calls to any of the date and time functions.
   The localtime_r() function does the same, but  stores  the  data  in  a
   user-supplied struct.  It need not set tzname, timezone, and daylight.

   The  asctime()  function  converts the broken-down time value tm into a
   null-terminated string with the same format  as  ctime().   The  return
   value   points   to  a  statically  allocated  string  which  might  be
   overwritten by subsequent calls to any of the date and time  functions.
   The  asctime_r()  function  does  the  same, but stores the string in a
   user-supplied buffer which should have room for at least 26 bytes.

   The mktime() function converts a broken-down time structure,  expressed
   as  local  time, to calendar time representation.  The function ignores
   the values supplied by the caller in the tm_wday  and  tm_yday  fields.
   The  value  specified in the tm_isdst field informs mktime() whether or
   not daylight saving time (DST) is in effect for the  time  supplied  in
   the  tm  structure: a positive value means DST is in effect; zero means
   that DST is not in effect; and a negative  value  means  that  mktime()
   should  (use  timezone  information and system databases to) attempt to
   determine whether DST is in effect at the specified time.

   The mktime() function modifies  the  fields  of  the  tm  structure  as
   follows:  tm_wday  and  tm_yday  are  set to values determined from the
   contents of the other fields; if structure members  are  outside  their
   valid  interval,  they  will  be  normalized  (so that, for example, 40
   October is changed into 9 November); tm_isdst is set (regardless of its
   initial  value)  to a positive value or to 0, respectively, to indicate
   whether DST is or is not in effect  at  the  specified  time.   Calling
   mktime()  also sets the external variable tzname with information about
   the current timezone.

   If the specified broken-down time cannot  be  represented  as  calendar
   time  (seconds  since the Epoch), mktime() returns (time_t) -1 and does
   not alter the members of the broken-down time structure.

RETURN VALUE

   On success, gmtime() and localtime() return a pointer to a struct tm.

   On success, gmtime_r() and localtime_r()  return  the  address  of  the
   structure pointed to by result.

   On success, asctime() and ctime() return a pointer to a string.

   On  success,  asctime_r()  and ctime_r() return a pointer to the string
   pointed to by buf.

   On success, mktime() returns  the  calendar  time  (seconds  since  the
   Epoch), expressed as a value of type time_t.

   On  error,  mktime()  returns  the  value  (time_t) -1.   The remaining
   functions return NULL on error.  On error, errno is set to indicate the
   cause of the error.

ERRORS

   EOVERFLOW
          The result cannot be represented.

ATTRIBUTES

   For   an   explanation   of   the  terms  used  in  this  section,  see
   attributes(7).

   ┌───────────────┬───────────────┬─────────────────────────────────┐
   │InterfaceAttributeValue                           │
   ├───────────────┼───────────────┼─────────────────────────────────┤
   │asctime()      │ Thread safety │ MT-Unsafe race:asctime locale   │
   ├───────────────┼───────────────┼─────────────────────────────────┤
   │asctime_r()    │ Thread safety │ MT-Safe locale                  │
   ├───────────────┼───────────────┼─────────────────────────────────┤
   │ctime()        │ Thread safety │ MT-Unsafe race:tmbuf            │
   │               │               │ race:asctime env locale         │
   ├───────────────┼───────────────┼─────────────────────────────────┤
   │ctime_r(),     │ Thread safety │ MT-Safe env locale              │
   │gmtime_r(),    │               │                                 │
   │localtime_r(), │               │                                 │
   │mktime()       │               │                                 │
   ├───────────────┼───────────────┼─────────────────────────────────┤
   │gmtime(),      │ Thread safety │ MT-Unsafe race:tmbuf env locale │
   │localtime()    │               │                                 │
   └───────────────┴───────────────┴─────────────────────────────────┘

CONFORMING TO

   POSIX.1-2001.   C89  and  C99  specify  asctime(),  ctime(),  gmtime(),
   localtime(),  and mktime().  POSIX.1-2008 marks asctime(), asctime_r(),
   ctime(), and ctime_r() as obsolete, recommending the use of strftime(3)
   instead.

NOTES

   The  four functions asctime(), ctime(), gmtime() and localtime() return
   a pointer to static data and hence are not  thread-safe.   The  thread-
   safe  versions,  asctime_r(),  ctime_r(), gmtime_r() and localtime_r(),
   are specified by SUSv2.

   POSIX.1-2001 says: "The asctime(), ctime(), gmtime(),  and  localtime()
   functions  shall  return values in one of two static objects: a broken-
   down time structure and an array of type char.  Execution of any of the
   functions  may  overwrite  the  information returned in either of these
   objects by any of the other functions."  This can occur  in  the  glibc
   implementation.

   In many implementations, including glibc, a 0 in tm_mday is interpreted
   as meaning the last day of the preceding month.

   The glibc version of struct tm has additional fields

          long tm_gmtoff;           /* Seconds east of UTC */
          const char *tm_zone;      /* Timezone abbreviation */

   defined when _BSD_SOURCE was set before including <time.h>.  This is  a
   BSD extension, present in 4.3BSD-Reno.

   According  to POSIX.1-2004, localtime() is required to behave as though
   tzset(3)  was  called,  while  localtime_r()   does   not   have   this
   requirement.   For  portable  code,  tzset(3)  should  be called before
   localtime_r().

SEE ALSO

   date(1), gettimeofday(2),  time(2),  utime(2),  clock(3),  difftime(3),
   strftime(3), strptime(3), timegm(3), tzset(3), time(7)

COLOPHON

   This  page  is  part of release 4.09 of the Linux man-pages project.  A
   description of the project, information about reporting bugs,  and  the
   latest     version     of     this    page,    can    be    found    at
   https://www.kernel.org/doc/man-pages/.

                              2016-12-12                          CTIME(3)





Opportunity


Personal Opportunity - Free software gives you access to billions of dollars of software at no cost. Use this software for your business, personal use or to develop a profitable skill. Access to source code provides access to a level of capabilities/information that companies protect though copyrights. Open source is a core component of the Internet and it is available to you. Leverage the billions of dollars in resources and capabilities to build a career, establish a business or change the world. The potential is endless for those who understand the opportunity.

Business Opportunity - Goldman Sachs, IBM and countless large corporations are leveraging open source to reduce costs, develop products and increase their bottom lines. Learn what these companies know about open source and how open source can give you the advantage.





Free Software


Free Software provides computer programs and capabilities at no cost but more importantly, it provides the freedom to run, edit, contribute to, and share the software. The importance of free software is a matter of access, not price. Software at no cost is a benefit but ownership rights to the software and source code is far more significant.


Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.





Free Books


The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.


Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.





Education


Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.


Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.