Content-type: text/html Man page of timezone


Section: C Library Functions (3)
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daylight, timezone, tzname, tzset - Sets and accesses timezone conversion information  


Standard C Library (, libc.a)  


#include <time.h>

void tzset(void):

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


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

tzset():  POSIX.1, XPG4, XPG4-UNIX

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 timezone information. On Tru64 UNIX systems, this information is located in the /etc/zoneinfo/localtime file. See the section, Tru64 UNIX Timezone Handling, for details.

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

        tzname[0] = "std";
        tzname[1] = "dst";

where std indicates the standard timezone and dst designates the alternative timezone (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:


Tru64 UNIX Timezone Handling

Tru64 UNIX uses a public-domain timezone-handling package that puts timezone conversion rules in easily accessible and modifiable files. These files are in the directory /etc/zoneinfo/sources. The timezone 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 timezone information.

The tzset() function uses the tzfile-formatted file linked by /etc/zoneinfo/localtime to set the timezone conversion information. The /etc/zoneinfo/localtime link is set during installation to a file in the /etc/zoneinfo directory. For example, in the eastern United States, /etc/zoneinfo/localtime is linked to /etc/zoneinfo/US/Eastern.

If the TZ environment variable is defined, the defined value overrides the timezone 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 Timezone Information

The libc ctime() and localtime() routines return the local time and timezone 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 timezone information, there are three relevant fields: A flag that is set to 1 if daylight savings time is currently 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 Timezone Information

The /etc/zoneinfo/localtime link can be changed by the system administrator to any file in the /etc/zoneinfo directory.

For example, to change the default system timezone to Canada's Atlantic time zone, enter the following as superuser:

# ln -s /etc/zoneinfo/Canada/Atlantic /etc/zoneinfo/localtime

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

If the timezone 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 timezone information by setting the TZ environment variable as described in the section, The TZ Environment 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:

    stdoffset[dst[offset] [,start[/time],end[/time]]] [Digital]  If TZ has the single colon format (:), Coordinated Universal Time (UTC) is used.

[Digital]  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 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:

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.  


[Digital] For users of the SVID2 habitat, TZ is defined by default in the following format:
    std offset [dst[offset] ] [Digital] 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.  


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



Tru64 UNIX Timezone Handling
Getting Timezone Information
Setting Timezone Information
The TZ Environment Variable

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Time: 02:40:48 GMT, October 02, 2010