Beginning this year, Daylight Saving Time in the United States begins earlier and runs later than in prior years. Under the new rules, Daylight Saving Time begins on the second Sunday in March and ends on the first Sunday in November. Previously, it began on the first Sunday in April and ended on the last Sunday in October. This change was signed into law as part of the Energy Policy Act of 2005.
This change means that any device which automatically changes its clock to match Daylight Saving Time, such as a VCR, either needs to be updated with new rules, or must have its clock changed manually on the affected dates. Apple included the new rules for Mac OS X 10.4 Tiger in the 10.4.6 update. (The 10.4.5 update also updated the Daylight Saving Time rules for changes in Australia and other locations.) Currently Apple has only released updates for Mac OS X 10.4 Tiger.
Turning the Hands -- Unless updates are issued for prior releases of Mac OS X, the clocks on computers running 10.3 or earlier will not show the correct time for three weeks in March and one week in November, in perpetuity. During those weeks, a number of things might go wrong. Messages created in Apple's Mail client (and probably others) will have the wrong timestamp, possibly resulting in users' messages being missed by their recipients. Events in iCal will display incorrectly, possibly causing people to miss appointments. Similarly, anyone collaborating on documents, and resolving changes based on timestamp, will be thrown askew. Authentication to network-based services (email, file servers, etc) might fail, as servers may refuse connection attempts if they appear to be too far outside the norm. (Kerberos servers, such as those available in Mac OS X Server, behave in this manner.)
In order to avoid these problems, folks using older releases will have to change their computers' clocks manually to the new "correct" time when Daylight Saving Time takes effect on 11-Mar-07, and then again on 01-Apr-07 (when those earlier versions of Mac OS X try to change it based on the old rules). Users will similarly have to adjust their computers' clocks on 28-Oct-07 and 04-Nov-07.
There are two options for updating clocks. If your computer uses a time server to set the date and time automatically, you can simply adjust the time zone (in the Time Zone pane of the Date & Time system preferences) to a zone that is an hour earlier or later, as appropriate. If your computer does not use a time server, you can simply adjust the time in the Date & Time pane of the Date & Time system preference. Either way, there may be problems with software that calculates time internally using Coordinated Universal Time (UT, also known as Greenwich Mean Time or GMT).
Apple's Responsibility -- We hope Apple will issue updates for Mac OS X 10.3 Panther and 10.2 Jaguar, else users will have to adjust their computers' clocks every year, twice on every Daylight Saving Time start and end date, for a total of four manual adjustments per year. An Apple representative declined to comment on "future plans or possible future software updates."
Unlike other operating system vendors, including Microsoft, Red Hat, and Sun, Apple has not posted sufficient information regarding how the change in Daylight Saving Time affects their products, nor which products are patched or unpatched. This situation is sadly familiar, for they likewise do not post life cycle support schedules for Mac OS X (again in contrast with Microsoft, Red Hat, and Sun), leaving customers to guess whether they can expect patches for security vulnerabilities. In this case, it's a simple matter of making sure the clock is right, and Apple's silent, de facto message of "upgrade to Tiger" is woefully inappropriate.
Other Software -- Some calendaring software may also require an update, as did Microsoft Entourage. The recent Microsoft Office for Mac 11.3.3 update fixed Entourage 2004's Daylight Saving Time rules. Microsoft told TidBITS that Entourage X would not be updated for the new Daylight Saving Time rules. In other words, if you use Entourage X for calendaring, you're really going to want to upgrade to Entourage 2004. (If you use Entourage with a Microsoft Exchange server, you should coordinate updates with your Exchange administrator, as Exchange must also be updated with the new rules.)
Happily, a fix for Mac OS X 10.3 Panther (both the desktop and server versions) has appeared in the form of an unofficial installer from Ian Ward Comfort of Stanford University that updates the necessary zoneinfo files and the ICU data archive to enable Cocoa applications like iCal to function correctly. You can also see Ian's shell script if you're concerned about running the installer. Finally, a Web site - DSTPatch.com - has sprung up to track available vendor patches; any system administrator or network administrator would do well to check it out.
If you're wondering why we bother with Daylight Saving Time at all (and different parts of the United States, along with various other countries, do not), you're not alone. The main rationale in the United States is energy conservation, but other stated benefits include increased opportunities for outdoor activities and fewer traffic injuries.