Mac Developer Library Developer


This manual page is part of Xcode Tools version 5.0

To obtain these tools:

If you are running a version of Xcode Tools other than 5.0, view the documentation locally:

  • In Xcode

  • In Terminal, using the man(1) command

Reading manual pages

Manual pages are intended as a quick reference for people who already understand a technology.

  • To learn how the manual is organized or to learn about command syntax, read the manual page for manpages(5).

  • For more information about this technology, look for other documentation in the Apple Developer Library.

  • For general information about writing shell scripts, read Shell Scripting Primer.

TZSET(3)                 BSD Library Functions Manual                 TZSET(3)

     tzset, tzsetwall -- initialize time conversion information

     Standard C Library (libc, -lc)

     #include <time.h>



     The tzset() function initializes time conversion information used by the library routine localtime(3).
     The environment variable TZ specifies how this is done.

     If TZ does not appear in the environment, the best available approximation to local wall clock time, as
     specified by the tzfile(5)-format file /etc/localtime, is used.

     If TZ appears in the environment but its value is a null string, Coordinated Universal Time (UTC) is
     used (without leap second correction).

     If TZ appears in the environment and its value begins with a colon (`:'), the rest of its value is used
     as a pathname of a tzfile(5)-format file from which to read the time conversion information.  If the
     first character of the pathname is a slash (`/'), it is used as an absolute pathname; otherwise, it is
     used as a pathname relative to the system time conversion information directory.

     If its value does not begin with a colon, it is first used as the pathname of a file (as described
     above) from which to read the time conversion information.  If that file cannot be read, the value is
     then interpreted as a direct specification (the format is described below) of the time conversion

     If the TZ environment variable does not specify a tzfile(5)-format file and cannot be interpreted as a
     direct specification, UTC is used.

     The tzsetwall() function sets things up so that localtime returns the best available approximation of
     local wall clock time.

     When TZ is used directly as a specification of the time conversion information, it must have the fol-lowing following
     lowing syntax (spaces inserted for clarity):

           std offset [dst [offset] [, rule]]


           std and dst  Three or more bytes that are the designation for the standard (std) or summer (dst)
                        time zone.  Only std is required; if dst is missing, then summer time does not apply
                        in this locale.  Upper and lowercase letters are explicitly allowed.  Any characters
                        except a leading colon (`:'), digits, comma (`,'), minus (`-'), plus (`+'), and
                        ASCII NUL are allowed.

           offset       Indicates the value one must add to the local time to arrive at Coordinated Univer-sal Universal
                        sal Time.  The offset has the form:


                        The minutes (mm) and seconds (ss) are optional.  The hour (hh) is required and may
                        be a single digit.  The offset following std is required.  If no offset follows dst,
                        summer time is assumed to be one hour ahead of standard time.  One or more digits
                        may be used; the value is always interpreted as a decimal number.  The hour must be
                        between zero and 24, and the minutes (and seconds) -- if present -- between zero and
                        59.  If preceded by a (`-') the time zone shall be east of the Prime Meridian; oth-erwise otherwise
                        erwise it shall be west (which may be indicated by an optional preceding (`+')).

           rule         Indicates when to change to and back from summer time.  The rule has the form:


                        where the first date describes when the change from standard to summer time occurs
                        and the second date describes when the change back happens.  Each time field
                        describes when, in current local time, the change to the other time is made.

                        The format of date is one of the following:

                        J n      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 the occasional February

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

                        M m.n.d  The d'th day (0 <= d <= 6) of week n of month m of the year (1 <= n <= 5),
                                 (1 <= m <= 12), where week 5 means ``the last d day in month m'' which may
                                 occur in either the fourth or the fifth week).  Week 1 is the first week in
                                 which the d'th day occurs.  Day zero is Sunday.

                                 The time has the same format as offset except that no leading sign (`-') or
                                 (`+') is allowed.  The default, if time is not given, is 02:00:00.

                        If no rule is present in the TZ specification, the rules specified by the
                        tzfile(5)-format file posixrules in the system time conversion information directory
                        are used, with the standard and summer time offsets from UTC replaced by those spec-
                        ified by the offset values in TZ.

     For compatibility with System V Release 3.1, a semicolon (`;') may be used to separate the rule from
     the rest of the specification.

     /etc/localtime                  local time zone file
     /usr/share/zoneinfo             time zone directory
     /usr/share/zoneinfo/posixrules  rules for POSIX-style TZ's
     /usr/share/zoneinfo/GMT         for UTC leap seconds

     If the file /usr/share/zoneinfo/GMT does not exist, UTC leap seconds are loaded from

     date(1), gettimeofday(2), ctime(3), getenv(3), time(3), tzfile(5)

     The tzset() and tzsetwall() functions first appeared in 4.4BSD.

BSD                            November 17, 1993                           BSD

Reporting Problems

The way to report a problem with this manual page depends on the type of problem:

Content errors
Report errors in the content of this documentation with the feedback links below.
Bug reports
Report bugs in the functionality of the described tool or API through Bug Reporter.
Formatting problems
Report formatting mistakes in the online version of these pages with the feedback links below.