Microsoft planned to send its daylight-saving patch to Windows PCs with the "automatic update" feature Tuesday. Users with automatic updates turned off should download the patch from Microsoft. (New machines running Windows Vista are immune, since Vista was finalized after the 2005 law passed.)
However, computers running anything older than the most recent version of Windows XP, known as Service Pack 2, no longer get this level of tech support.
Owners of those PCs should go into the control panel and unclick the setting that tells the machine to automatically change the clock for daylight-saving time. They have to make the change themselves when the moment arrives. (This is a sizable population; according to Gartner, Windows 2000 alone was still running 14 percent of PCs worldwide last year.)
For people who store their appointments in Microsoft Outlook or other desktop-based calendar programs -- rather than dynamic, Web-based programs such as Google Calendar -- the situation gets trickier. Patches for calendar programs are available, but appointments entered before a patch was applied might still be registered in standard time rather than daylight time -- off by an hour.
Microsoft advises heavy calendar users to go online and download a small program known as "tzmove" -- Time Zone Move -- that can retrofit all previously booked appointments to the new daylight-saving rules. Other vendors offer similar tools for their systems.
Of course, it's likely not everyone would take that step, said Rich Kaplan, a Microsoft customer service vice president who oversaw the company's Y2K efforts and heads daylight-saving preparations. Hence Microsoft's advice to be cautious about meetings between March 11 and April 1.
"Because if one person applied the update, and one person didn't," he said, "you could end up there at the wrong time."