This article helps you to understand various time zone formats, interpret the Portable Operating System Interface (POSIX) format, and understand the output of the zdump command with examples.

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Rajalakshmi Srinivasaraghavan (rajalakshmi@in.ibm.com), AIX Developer, IBM China

Rajalakshmi Srinivasaraghavan is a developer in the IBM AIX team. She has worked on support and development projects on AIX in areas such as TCP/IP, security, commands, and more for 6.7 years. She holds a BE degree in Computer Science.



13 December 2012

Introduction

A time zone can be specified in two formats: POSIX and Olson. This article helps you to understand the Olson and POSIX time zone formats and helps you set the required time zone values in IBM® AIX®.


POSIX format specification

The POSIX time zone format is the traditionally used format for AIX systems and provides a slight performance advantage over the Olson time zone format. Example of a POSIX format is EST5EDT.

The advantage of POSIX is that you can easily and explicitly specify the time zone and daylight saving time (DST) details manually, however you wish. The performance of applications that call time functions will be faster than using Olson specification. And whenever a nation's government decides to change its DST rules, the POSIX format is simpler because we can simply change the variable definition. There is no need to install any new patch to update time database files, as Olson requires.

The disadvantage of this approach is that it cannot track the history of timezone-related changes and it is not easy to read as it looks cryptic. When a government changes the rules and you update your time zone (TZ) variable, it is assumed to be the same DST rule for all years past and future.


Olson format specification

This method maintains a database for each time zone under the /usr/share/lib/zoneinfo directory in AIX. The advantage with this approach is that the time zone name is easy to specify and Olson maintains a history of time zone and DST-related changes.

This format uses known names of cities or regions. We can specify the time zone name in a simple, easy-to-understand format, such as "America/Sao_Paulo" rather than specifying the more complex TZ=GRNLNDST3GRNLNDDT,M10.3.0/00:00:00,M2.4.0/00:00:00.

The disadvantage of this approach is that Olson has to load the database file for each time zone specified, and then parse it to find the time zone and DST details. This process can have a modest performance penalty compared to the POSIX format. Additionally, when a government changes a time zone or the DST rule, a new patch becomes necessary for the Olson time database file.


Understanding the POSIX format

The TZ variable specified in POSIX format contains all the information required to identify the time zone, specify when to switch DST on and off, and specify the offset from Coordinated Universal Time (UTC). The system internally does everything in UTC time, and any display of time to the users is a computed offset, depending on the time zone and DST rules specified.

The format is TZ = local_timezone,date/time,date/time.

Here, date is in the Mm.n.d format, where:

  • Mm (1-12) for 12 months
  • n (1-5) 1 for the first week and 5 for the last week in the month
  • d (0-6) 0 for Sunday and 6 for Saturday

For example:

    		 TZ=CST6CDT,M3.2.0/2:00:00,M11.1.0/2:00:00

This would effect a change to daylight saving time at 2:00 AM on the second Sunday in March and change back at 2:00 AM on the first Sunday in November, and keep 6 hours time offset from Greenwich mean time (GMT) every year. The breakdown of the string is:

  • CST6CDT is the name of the time zone
  • CST is the abbreviation used when DST is off
  • 6 hours is the time difference from GMT
  • CDT is the abbreviation used when DST is on
  • ,M3 is the third month
  • .2 is the second occurrence of the day in the month
  • .0 is Sunday
  • /2:00:00 is the time
  • ,M11 is the eleventh month
  • .1 is the first occurrence of the day in the month
  • .0 is Sunday
  • /2:00:00 is the time

Customizing DST using the POSIX format time zone

The POSIX time format can be customized to override the default daylight savings time values. The customized override values for DST can be used when the default POSIX time zone change is not expected on the US dates.

If the DST option is enabled, the default in AIX is for the system time to move forward by 1 hour (to DST) at 2:00 a.m. the second Sunday in March, and move back one hour (to Standard Time) at 2:00 a.m. on the first Sunday in November .However, the date and time at which the switch to DST occurs can be customized by root (global environment) or by users (user environment) by setting the $TZ environment variable.

For example, to change the DST change from March to April with daylight saving time of 30 minutes, you need to enter:

TZ=CET6CEST5 30,M4.5.0/02:00:00,M10.5.0/03:00:00

Here, the difference between 6 and 5:30 is calculated for daylight saving time.

The following examples represent some of the customized POSIX formats:

HAST10HADT,M4.2.0/03:0:0,M10.2.0/03:0:00
AST9ADT,M3.2.0,M11.1.0
AST9ADT,M3.2.0/03:0:0,M11.1.0/03:0:0
EST5EDT,M3.2.0/02:00:00,M11.1.0/02:00:00
GRNLNDST3GRNLNDDT,M10.3.0/00:00:00,M2.4.0/00:00:00
EST5EDT,M3.2.0/02:00:00,M11.1.0
EST5EDT,M3.2.0,M11.1.0/02:00:00
CST6CDT,M3.2.0/2:00:00,M11.1.0/2:00:00
MST7MDT,M3.2.0/2:00:00,M11.1.0/2:00:00
PST8PDT,M3.2.0/2:00:00,M11.1.0/2:00:00

Managing the TZ variable

The TZ environment variable is used to represent time zone information. The value of TZ can be specified either in the POSIX or Olson format.

To set the time zone using Olson-defined values, use the following System Management Interface Tool (SMIT) path:

System Environments > Change / Show Date, Time and Time Zone.

To set the Olson format in the command line, enter:

#TZ=America/New_York

If the TZ environment variable does not match a proper Olson time zone value, AIX then follows the POSIX specification rules.

To set the POSIX format in the command line, enter:

#TZ=EST5EDT

If the value set is invalid or unrecognized, the time zone defaults to UTC.

To determine the time zone format in the command line, enter:

# echo $TZ

.


Output of the zdump command

The output of the zdump command gives a clear picture of DST change timing for the particular time zone. For example, in Figure 1, the first two lines explain the scenario when daylight saving is enabled and last two lines explain the scenario when it is disabled.

zdump command output
zdump command output

Line 1:

On Sunday, 11 March, exactly after 01:59:59 clock is expected to set forward by one hour. The abbreviation is EST at 01:59:59 and isdst=0 specifies that DST is off at this time. The corresponding UTC time is 06:59:59.

Line 2:

One second after 01:59:59, time changes to 03:00:00, DST is on and the abbreviation changes to EDT. isdst=1 specifies that DST is enabled. The corresponding UTC time is 07:00:00.

Line 3:

On Sunday, 4 November, exactly after 01:59:59, clock is expected to set back one hour. The abbreviation is EDT at 01:59:59 and isdst=1 specifies that DST is on at this time. The corresponding UTC time is 05:59:59.

Line 4:

One second after 01:59:59, time changes to 01:00:00, DST is off and the abbreviation changes to EST. isdst=0 specifies that DST is disabled. The corresponding UTC time is 06:00:00.


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