timezone(3)						     Library Functions Manual						       timezone(3)

NAME

       daylight, timezone, tzname, tzset - Sets and accesses time zone conversion information

LIBRARY

       Standard C Library (libc.so, libc.a)

SYNOPSIS

       #include <time.h>

       void tzset(void):

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

STANDARDS

       Interfaces documented on this reference page conform to industry standards as follows:

       tzset():  POSIX.1, XSH4.2

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

DESCRIPTION

       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 details.

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

	       tzname[0] = "std";
	       tzname[1] = "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, The 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
       example:

								     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 for-
       mat 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,  The  TZ  Environment  Variable,  for
       details.

   Getting Time Zone Information
       The  libc ctime() and localtime() routines return the local time and time zone information.  The ctime() routine returns a string that cor-
       responds 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 flag that is set to 1 if daylight savings time is cur-
       rently in effect.  Otherwise, the flag is set to 0.  Seconds east of Greenwich.	For example, -18000  means  5  hours  west  of	Greenwich.
       Abbreviation 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 administrator 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, The  TZ  Environ-
       ment Variable.

   The 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 Universal 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 pathname beginning with a slash (/) represents an absolute pathname; other-
       wise, the pathname is relative to the system time conversion information 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 lower-case 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:
	    hh[:mm[:ss]]

	      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 explicitly refer to February 29.	The  zero-
	      based  Julian day  (0 <= n <= 365).  Leap days are counted making it possible to refer to February 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.

NOTES

       [Tru64 UNIX]  For users of the SVID2 habitat, TZ is defined by default in the following format:
	   std offset [dst[offset] ] [Tru64 UNIX]  For users of the SVID3 habitat, TZ is defined by default in the following format:
	   :pathname See the section, The TZ Environment Variable, for definitions of the parameters used in these formats.

RELATED INFORMATION

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

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

       Files: tzfile(4)

       Standards: standards(5) delim off

																       timezone(3)