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NAME    [Toc]    [Back]

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

SYNOPSIS    [Toc]    [Back]

       #include <time.h>

       void tzset(void);

       extern int daylight; extern  long  timezone;  extern  char

LIBRARY    [Toc]    [Back]

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

STANDARDS    [Toc]    [Back]

       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    [Toc]    [Back]

       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

       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 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    [Toc]    [Back]
       The  operating  system uses a public-domain time zone handling
 package that puts time zone conversion rules in easily
  accessible  and modifiable 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

   Getting Time Zone Information    [Toc]    [Back]
       The libc routines ctime() and localtime() return the local
       time  and  time  zone  information.  The  ctime()  routine
       returns a string that corresponds 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 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    [Toc]    [Back]
       The  /etc/zoneinfo/localtime  link  can  be changed by the
       system administrator to  any  file  in  the  /etc/zoneinfo

       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 Variable.

   TZ Environment Variable    [Toc]    [Back]
       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) format file from which to read the
       time conversion information.  A pathname beginning with  a
       slash  (/) represents an absolute pathname; otherwise, 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 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


              The minutes (mm) and seconds (ss) are optional. The
              hour (hh) is required and can be either one or  two
              digits. The offset following 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    [Toc]    [Back]

       [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 parameters used in these formats.

SEE ALSO    [Toc]    [Back]

       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)

[ Back ]
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