Since the Germans first started doing it in 1916, much of the industrialized world has adopted a system of shifting time by an hour twice per year. In Europe, this is generally known as "summer time," but in the United States and Canada it's known as daylight saving time (DST). Never shy about screwing with a perfectly workable system, the US Congress passed a law in 2005, which goes into effect this year, that changes the dates when we switch to and away from DST.

This change has many implications that the original authors of the law probably didn't think of. It's easy enough to change ordinary clocks, but many other devices keep time and will need to be either adjusted or patched by the manufacturers. For example, in my home there are two TiVos, a dozen or so computers, several wireless access points and routers, a VCR, and a PBX. All of these devices automatically adjust their clocks for DST now, but they'll need to be updated because this year DST begins earlier (March 11 instead of April 1) and ends later (November 4 versus October 28) than it has before.

The 2007 DST change poses an especially interesting circumstance for Exchange administrators. There are actually four sets of changes required:

  • patches for Windows, including Windows Server 2003, Windows 2000 Server, Windows XP, and Windows Mobile
  • patches for Exchange, which are required for OWA and other programs that use the Collaboration Data Objects (CDO) libraries
  • patches for Outlook, Microsoft Entourage, and other clients that create and process calendar data
  • updates to existing appointments that fall during the new extended DST period (March 11–April 1 and October 28–November 4)

So, what do you actually need to do? To make sure your environment is prepared for the DST changes, you need to begin by patching your Windows servers so that they'll adjust their clocks automatically at the appropriate times. The Microsoft article "2007 time zone update for Microsoft Windows operating systems" describes the process required to apply updates to Windows Server 2003 and Windows XP Service Pack 2 (SP2). Windows Vista already understands the extended DST period; Windows 2000 customers with extended support agreements will be able to get a hotfix. Users on Windows XP SP1 or Windows NT are out of luck.

Next, you should patch your client OSs. Ideally, you'd do this at the same time as the server OS patches. Don't forget your Windows Mobile clients! (I'll explain why next week.) After these patches are in place, Microsoft recommends patching Exchange Server 2003 by using the patch described in the Microsoft article "Update for daylight saving time changes in 2007 for Exchange 2003". Exchange Server 2007 already includes the patch; if you’re still on Exchange 2000 Server or Exchange Server 5.5, you can get a patch from Microsoft if you’re under an extended support contract. After Exchange is patched, the real fun starts: You have to update calendar and appointment data on your servers. That process deserves its own column, so I'll write about it next week.

In the meantime, the Web page Preparing for daylight saving time changes in 2007 is Microsoft's master reference for the whole DST update. It contains this somewhat frightening sentence in the introduction: "Please check this page at least weekly to ensure you have the latest information available." So, your homework before next week's UPDATE is to visit that page at least once!