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Observing Daylight Saving Time (DST)

by Curtis Smith

Starting in April 2006, Indiana will begin observing daylight saving time (DST), something that hasn't happened since October 1970. In the time between 1970 and 2005, Indiana has skipped the time changes, essentially keeping Indiana at a constant five hours from Greenwich Mean Time (GMT) throughout the year, always being on Eastern Standard Time (EST). After 2005, most of Indiana will move to Eastern Standard Time, with daylight saving time in the summer months, putting Indiana in Eastern Daylight Saving Time (EDT), four hours from Greenwich Mean Time.

In preparation for the April change, most computers may need to be tweaked so that they display the correct time.

Adjusting Timezones For UNIX

Not all time zones are created equal. Even though Indiana is on Eastern Standard Time, we haven't observed the change to daylight saving time since 1970, so we need to set our computers to have a special timezone that reflects an unusual way to handle the timezone offset from Greenwich Mean Time.

For UNIX computers (including Linux, FreeBSD and MacOS X), there are a number of choices that will be correct for our timezone. The three best choices are:

  • America/Indiana/Indianapolis,
  • America/Indianapolis, or
  • US/East-Indiana

It is not correct to use EST5DST or Eastern Time. These timezones include daylight saving time adjustments that did not apply between 1970 and 2005.

To illustrate the difference between setting America/Indiana/Indianapolis and Eastern Time as the timezone, consider the following:

UNIX uses Greenwich Mean Time internally for all timestamps, such as the last modification time of a file. Before UNIX displays the last modification time of a file to the screen, it first converts the timestamp from GMT to the current timezone. A file with a last modification time from last summer would look like this:

$ ls -l ski_report
-rw-r--r--  1 cs  cs  9566 Jun 30 14:30 ski_report

However, if we use a straight EST timezone instead of an Indiana timezone, the last modification time would look like this:

$ ls -l ski_report
-rw-r--r--  1 cs  cs  9566 Jun 30 15:30 ski_report

An hour off!! That's because last summer, Indiana did not use daylight saving time, but everyone else in the Eastern timezone did.

In order to have the UNIX computers ready for timezone changes in April, most will need updating to have a compiled timezone file that includes the daylight saving time adjustment for Indiana in 2006.

This official site holding timezone information files is This site contains source timezone information files and open source software that manages timezone offset calculations.

Download the file named tzdata2006a.tar.gz (or any newer files), and extract the file containing information about Indiana's timezone in the file named northamerica. It is this file that will need to be compiled and installed so that the America/Indiana/Indianapolis timezone file contains the new 2006 daylight saving time changes.

Check your operating system's documentation on how to compile new timezone files. One place to start looking is the manual page for the command zic. For most systems, executing the following command as superuser will update the North America timezone files with the latest information:

# zic northamerica

Other useful places to get more information:

Adjusting Timezones for Windows

Windows uses a simple timezone setting that has two parts: (1) the timezone and (2) whether or not to automatically adjust clock for daylight saving time changes. Currently, most Windows computer would be set to the timezone (GMT-05:00) Indiana (East). To adjust for Indiana's observance of daylight saving time, change the timezone to (GMT-05:00) Eastern Time (US & Canada). There should be a check in the checkbox at the bottom on the timezone properties screen.

More information is available about potential disruption to calendars caused by Indiana's switch to observing Daylight Saving Time. Refer to the ECN Knowledge Base article Windows DST Problems for more information.

Adjust Timezones for Macintosh

As an additional bit of information for Macintosh users:

During a meeting of the Purdue IT administrators, it came to our attention that Mac OS X, version 10.4.5, has a patch for the east Indiana timezone. But users with MacOS X versions 10.3 and earlier cannot get a timezone patch from Apple without an update to version 10.4.

Even though it might be possible to download and install the zic files, like other UNIX hosts, the Mac OS desktop uses a separate set of data files to control timezone adjustments for some desktop utilities, including the menu bar.

See Apple support article 303179 for more information.

Last modified: 2006/08/09 12:59:0.837000 GMT-4 by Unknown
Created: 2006/08/09 12:59:0.837000 GMT-4 by brian.r.brinegar.1.



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