tzset(3)
NAME
tzset, tzname, timezone, daylight - initialize time conversion information
SYNOPSIS
#include <time.h>
void tzset (void);
extern char *tzname[2];
extern long timezone;
extern int daylight;
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
tzset(): _POSIX_C_SOURCE
tzname: _POSIX_C_SOURCE
timezone, daylight: _XOPEN_SOURCE
|| /* Glibc since 2.19: */ _DEFAULT_SOURCE
|| /* Glibc versions <= 2.19: */ _SVID_SOURCE
DESCRIPTION
The tzset() function initializes the tzname variable from the TZ environment variable. This function is automatically called by the other time conversion functions that depend on the timezone. In a System-V-like environment, it will also set the variables timezone (seconds West of UTC) and daylight (to 0 if this timezone does not have any daylight saving time rules, or to nonzero if there is a time, past, present or future when daylight saving time applies). If the TZ variable does not appear in the environment, the system timezone is used. The system timezone is configured by copying, or linking, a file in the tzfile(5) format to /etc/localtime. A timezone database of these files may be located in the system timezone directory (see the FILES section below). If the TZ variable does appear in the environment, but its value is empty, or its value cannot be interpreted using any of the formats specified below, then Coordinated Universal Time (UTC) is used. The value of TZ can be one of two formats. The first format is a string of characters that directly represent the timezone to be used: std offset[dst[offset][,start[/time],end[/time]]] There are no spaces in the specification. The std string specifies an abbreviation for the timezone and must be three or more alphabetic characters. When enclosed between the less-than (<) and greater-than (>) signs, the characters set is expanded to include the plus (+) sign, the minus (-) sign, and digits. The offset string immediately follows std and specifies the time value to be added to the local time to get Coordinated Universal Time (UTC). The offset is positive if the local timezone is west of the Prime Meridian and negative if it is east. The hour must be between 0 and 24, and the minutes and seconds 00 and 59: [+|-]hh[:mm[:ss]] The dst string and offset specify the name and offset for the corresponding daylight saving timezone. If the offset is omitted, it defaults to one hour ahead of standard time. The start field specifies when daylight saving time goes into effect and the end field specifies when the change is made back to standard time. These fields may have the following formats: Jn This specifies the Julian day with n between 1 and 365. Leap days are not counted. In this format, February 29 can't be represented; February 28 is day 59, and March 1 is always day 60. n This specifies the zero-based Julian day with n between 0 and 365. February 29 is counted in leap years. Mm.w.d This specifies day d (0 <= d <= 6) of week w (1 <= w <= 5) of month m (1 <= m <= 12). Week 1 is the first week in which day d occurs and week 5 is the last week in which day d occurs. Day 0 is a Sunday. The time fields specify when, in the local time currently in effect, the change to the other time occurs. If omitted, the default is 02:00:00. Here is an example for New Zealand, where the standard time (NZST) is 12 hours ahead of UTC, and daylight saving time (NZDT), 13 hours ahead of UTC, runs from the first Sunday in October to the third Sunday in March, and the changeovers happen at the default time of 02:00:00: TZ="NZST-12:00:00NZDT-13:00:00,M10.1.0,M3.3.0" The second format specifies that the timezone information should be read from a file: :[filespec] If the file specification filespec is omitted, or its value cannot be interpreted, then Coordinated Universal Time (UTC) is used. If filespec is given, it specifies another tzfile(5)-format file to read the timezone information from. If filespec does not begin with a '/', the file specification is relative to the system timezone directory. If the colon is omitted each of the above TZ formats will be tried. Here's an example, once more for New Zealand: TZ=":Pacific/Auckland"
ENVIRONMENT
TZ If this variable is set its value takes precedence over the
system configured timezone.
TZDIR If this variable is set its value takes precedence over the
system configured timezone database directory path.
FILES
/etc/localtime
The system timezone file.
/usr/share/zoneinfo/
The system timezone database directory.
/usr/share/zoneinfo/posixrules
When a TZ string includes a dst timezone without anything
following it, then this file is used for the start/end rules.
It is in the tzfile(5) format. By default, the zoneinfo
Makefile hard links it to the America/New_York tzfile.
Above are the current standard file locations, but they are
configurable when glibc is compiled.
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7). ┌──────────┬───────────────┬────────────────────┐ │Interface │ Attribute │ Value │ ├──────────┼───────────────┼────────────────────┤ │tzset() │ Thread safety │ MT-Safe env locale │ └──────────┴───────────────┴────────────────────┘
CONFORMING TO
POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
NOTES
4.3BSD had a function char *timezone(zone, dst) that returned the name of the timezone corresponding to its first argument (minutes West of UTC). If the second argument was 0, the standard name was used, otherwise the daylight saving time version.
SEE ALSO
date(1), gettimeofday(2), time(2), ctime(3), getenv(3), tzfile(5)
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/.
2016-03-15 TZSET(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.
