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timezone(3)							   timezone(3)

       daylight, timezone, tzname, tzset - sets and accesses time zone conver‐
       sion information

       #include <time.h>

       void tzset(void);

       extern int daylight; extern long timezone; extern char *tzname[];

       Standard C Library (, libc.a)

       Interfaces documented on this reference page conform to industry	 stan‐
       dards as follows:

       tzset():	 POSIX.1, XSH4.2

       Refer  to  the  standards(5)  reference page for more information about
       industry standards and associated tags.

       The tzset() function uses the value of the environment variable	TZ  to
       set  time  conversion  information  used	 by  several  other functions,
       including  ctime(),  ctime_r(),	getdate(),  getdate_r(),  localtime(),
       localtime_r(), mktime(), strftime(), and strptime().

       If  the	TZ  variable is not set, tzset() uses implementation-dependent
       default time zone information.  This  information  is  located  in  the
       /etc/zoneinfo/localtime	file.  See  the section Time Zone Handling for

       The tzset() function sets the external variable tzname as follows:

       tzname0 = "std"; tzname1 = "dst";

       where std indicates the standard	 time  zone  and  dst  designates  the
       alternative time zone (such as Daylight Savings Time). (These variables
       are described below in the section TZ Environment Variable.)

       The tzset() function also sets the external variable daylight to	 0  if
       Daylight	 Savings Time conversions should never be applied for the time
       zone in use. Otherwise, daylight is set to a nonzero value.

       The external variable timezone is set to the  difference,  in  seconds,
       between	Coordinated Universal Time (UTC) and local standard time.  For

       TZ    timezone
       EST   5*60*60
       GMT   0*60*60
       JST   -9*60*60
       MET   -1*60*60
       MST   7*60*60

       PST   8*60*60

   Time Zone Handling
       The operating system uses a public-domain time  zone  handling  package
       that  puts  time zone conversion rules in easily accessible and modifi‐
       able files.  These files are in	the  directory	/etc/zoneinfo/sources.
       The  time zone compiler zic(8) converts these files to a special format
       described in tzfile(4) and places them in the /etc/zoneinfo  directory.
       This  format  is	 readable  by the C library functions that handle time
       zone information.

       The  tzset()  function  uses  the  tzfile-formatted  file   linked   by
       /etc/zoneinfo/localtime	to  set	 the time zone conversion information.
       The /etc/zoneinfo/localtime link is set during installation to  a  file
       in  the /etc/zoneinfo directory. For example, for time zone information
       consistent with the  city  of  New  York	 on  the  American  continent,
       /etc/zoneinfo/localtime is linked to /etc/zoneinfo/America/New_York.

       If  the TZ environment variable is defined, the defined value overrides
       the time zone information in /etc/zoneinfo/localtime.  TZ can be set by
       a  user	as  a regular environment variable for converting to alternate
       time zones. See the section TZ Environment Variable for details.

   Getting Time Zone Information
       The libc routines ctime() and localtime() return	 the  local  time  and
       time zone information. The ctime() routine returns a string that corre‐
       sponds to the local time; for example, Tue Oct 27 13:35:29 1992.

       The localtime() routine returns a pointer to a tm structure (defined in
       <sys/time.h>)  that  contains the local time expressed in fields of the
       tm structure.  For time zone  information,  there  are  three  relevant
       fields: A option that is set to 1 if daylight savings time is currently
       in effect. Otherwise, the option is set to 0.  Seconds east  of	Green‐
       wich.   For example, -18000 means 5 hours west of Greenwich.  Abbrevia‐
       tion for the current time zone (for example, EST, PDT, GMT).

   Setting Time Zone Information
       The /etc/zoneinfo/localtime link can be changed by the system  adminis‐
       trator to any file in the /etc/zoneinfo directory.

       For  example,  the  following command changes the local time zone to be
       consistent with the city of New York on the American continent:

       # ln -sf /etc/zoneinfo/America/New_York /etc/zoneinfo/localtime

       Subsequent calls to the time zone related functions  in	libc  (ctime()
       and localtime()) use this link for the default time zone information.

       If  the	time  zone  and	 daylight  savings  time  information  in  the
       /etc/zoneinfo/sources directory is incorrect for your  time  zone,  you
       can  change  the	 information  in the source files and then use the zic
       command to generate a corresponding /etc/zoneinfo file.

       A user can override the default time zone information by setting the TZ
       environment  variable  as described in the section TZ Environment Vari‐

   TZ Environment Variable
       When TZ appears in the environment and its value is not a null  string,
       the value has one of three formats:

       : :pathname stdoffset[dst[offset][,start[/time],end[/time]]]

       [Tru64  UNIX]  If  TZ has the single colon format (:), Coordinated Uni‐
       versal Time (UTC) is used.

       [Tru64 UNIX]  If TZ has	the  colon-pathname  format  (:pathname),  the
       characters following the colon specify the pathname of a tzfile(4) for‐
       mat file from which to read the time conversion information.   A	 path‐
       name beginning with a slash (/) represents an absolute pathname; other‐
       wise, the pathname is relative to the system time  conversion  informa‐
       tion directory /etc/zoneinfo.

       If TZ does not begin with a colon (:), the components of the string are
       as follows: Three or more characters that are the designation  for  the
       standard (std) or alternative (dst) time zone (such as Daylight Savings
       Time). Only std is required. If dst is not  supplied,  the  alternative
       time  does  not	apply  to the locale. Upper- and lowercase letters are
       explicitly allowed. Any characters, except digits, a leading colon (:),
       comma  (,), minus (-), plus (+), and ASCII NUL, are allowed.  Indicates
       the value to be added to the local time to arrive at  GMT.  The	offset
       has the form:


	      The minutes (mm) and seconds (ss) are optional. The hour (hh) is
	      required and can be either one or two digits. The offset follow‐
	      ing  std	is required. If no offset follows dst, the alternative
	      time is assumed to be one hour ahead of standard	time.  One  or
	      more  digits  can	 be used; the value is always interpreted as a
	      decimal number.  The hour value must be between zero and 24. The
	      value  for  the minutes and seconds, if present, must be between
	      zero and 59. If preceded by a minus sign (-), the time  zone  is
	      east  of	the Prime Meridian; otherwise it is west, which can be
	      indicated by a preceding	plus  sign  (+).   Indicates  when  to
	      change  to and return from alternative time.  The start argument
	      is the date when the change from standard	 to  alternative  time
	      occurs;  end is the date for changing back. If start and end are
	      not specified, the default is the US Daylight Saving Time	 start
	      and  end	dates. The format for start and end must be one of the
	      following: The Julian day n (1 <= n <= 365). Leap days  are  not
	      counted.	That  is, in all years, including leap years, February
	      28 is day 59 and March 1 is day 60. It is impossible to  explic‐
	      itly  refer to February 29.  The zero-based Julian day (0 <=n <=
	      365). Leap days are counted making it possible to refer to  Feb‐
	      ruary 29.	 The dth day (0 <= d <= 6) of week n of month m of the
	      year (1 <= n <= 5, 1 <= m <= 12). When n is 5, it refers to  the
	      last  d  day  of month m which may occur in either the fourth or
	      fifth week. Week 1 is the	 first	week  in  which	 the  dth  day
	      occurs. Day zero is Sunday.  Describes the time when, in current
	      time, the change to or return from alternative time occurs.  The
	      time  parameter has the same format as offset, except that there
	      can be no leading minus (-) or plus (+) sign.  If	 time  is  not
	      specified, the default is 02:00:00.

       As  an example, the TZ variable value EST5EDT4,M4.1.0,M10.5.0 describes
       the rule defined in 1987 for the Eastern	 time  zone  in	 the  US.  EST
       (Eastern	 Standard Time) is the designation for standard time, which is
       5 hours behind GMT. EDT (Eastern Daylight Time) is the designation  for
       alternative  time, which is 4 hours behind GMT. EDT starts on the first
       Sunday in April and ends on the last Sunday in October. In both	cases,
       since  time  was	 not specified, the changes occur at the default time,
       which is 2:00 A.M. Note that the start and end dates did not need to be
       specified since they are the defaults.

       [Tru64  UNIX]  For users of the SVID2 habitat, TZ is defined by default
       in the following format:


       [Tru64 UNIX]  For users of the SVID3 habitat, TZ is defined by  default
       in the following format:


       See  the section TZ Environment Variable for definitions of the parame‐
       ters used in these formats.

       Functions: ctime(3), ctime_r(3), difftime(3), getdate(3), getdate_r(3),
       getenv(3),  localtime(3), localtime_r(3), mktime(3), strftime(3), strp‐
       time(3), time(3)

       Commands: date(1), zdump(8), zic(8)

       Files: tzfile(4)

       Standards: standards(5)


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