Topics: AIX, System Admin

Olson time zone support

"The public-domain time zone database contains code and data that represent the history of local time for many representative locations around the globe. It is updated periodically to reflect changes made by political bodies to time zone boundaries, UTC offsets, and daylight-saving rules. This database (often called tz or zoneinfo) is used by several implementations.

Each location in the database represents a national region where all clocks keeping local time have agreed since 1970. Locations are identified by continent or ocean and then by the name of the location, which is typically the largest city within the region. For example, America/New_York represents most of the US eastern time zone; America/Phoenix represents most of Arizona, which uses mountain time without daylight saving time (DST); America/Detroit represents most of Michigan, which uses eastern time but with different DST rules in 1975; and other entries represent smaller regions like Starke County, Indiana, which switched from central to eastern time in 1991 and switched back in 2006."

The public-domain time zone database is also widely known as the Olson time zone database and is the architecture on which the International Components for Unicode (ICU) and the Common Locale Data Repository (CLDR) time zone support relies.

In previous AIX releases, the method by which the operating system supports time zone conventions is based on the POSIX time zone specification. In addition to this industry standard approach, AIX V6.1 recognizes and processes the Olson time zone naming conventions to facilitate support for a comprehensive set of time zones.

This enhancement leverages the uniform time zone naming convention of the Olson database to offer an intuitive set of time zone values that can be assigned to the TZ time zone environment variable.

Note: Time zone definitions conforming to the POSIX specification are still supported and recognized by AIX. AIX checks the TZ environment variable to determine if the environment variable follows the POSIX specification rules. If the TZ environment variable does not match the POSIX convention, AIX calls the ICU library to get the Olson time zone translation.

The use of the Olson database for time zone support within AIX provides significant advantages over the traditional POSIX rules. One of the biggest advantages is that Olson database maintains a historical record of what the time zone rules were at given points in time, so that if the rules change in a particular location, dates and times can be interpreted correctly both in the present and past. A good example of this is the US state of Indiana, which just began using daylight saving time in the year 2006. Under the POSIX implementation, Indiana would have to set its time zone value to EST5EDT, which would format current dates correctly using daylight saving time, but would also format times from previous years as though they were on daylight saving time, which is incorrect. Use of the ICU API set for time zones also allows for localized display names for time zones. For example, Central Daylight Saving Time would have an abbreviation of CDT for all locales under a POSIX implementation, but under ICU/Olson, it displays properly as HAC (Heure Avancée du Centre) in a French locale.

As in previous AIX releases, system administrators can rely on the Systems Management Interface Tool (SMIT) to configure the time zone by using system defined values for the TZ environment variable. To accomplish this task, enter the main SMIT menu and select System Environments, Change / Show Date and Time to access the Change Time Zone Using System Defined Values menu. Alternatively, the SMIT fast path chtz_date will directly open the Change / Show Date and Time menu. Selecting the Change Time Zone Using System Defined Values option will prompt SMIT to open the Select COUNTRY or REGION menu.

SMIT uses the undocumented /usr/lib/nls/lstz -C command to produce the list of available countries and regions. Note that undocumented commands and features are not officially supported for customer use, are not covered by the AIX compatibility statement, and may be subject to change without notice.

After you have chosen the country or region in the Select COUNTRY or REGION menu, a new selection menu will list all available time zones for the country or region in question.

The selected value of the first column will be passed by SMIT to the chtz command, which in turn will change the TZ variable value in the /etc/environment system level configuration file. As with previous AIX releases, time zone configuration changes always require a system reboot to become effective.

SMIT uses the internal /usr/lib/nls/lstz -c command to produce the list of available time zones for a given country and region. The -c flag uses a country or region designation as the input parameter. The /usr/lib/nls/lstz -C command provides a list of available input parameters. The /usr/lib/nls/lstz command used without any flag provides a full list of all Olson time zones available on AIX. Note that undocumented commands and features are not officially supported for customer use, are not covered by the AIX compatibility statement, and may be subject to change without notice.

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