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Your questions: “How does Windows handle historical events with daylight saving time updates?”

I received a question from a customer who had applied the Windows OS update for daylight saving time in 2007 (kb 931836). They said that they noticed that after the update was applied, events in past years also adjucted to the new DST rules as well. “How does Windows handle historical events in software applications with daylight saving time updates?”


We will release a brief document that explains how documents and computers use and display time, and how Windows is impacted. Until then, here’s a brief explanation.


Overall, Windows software applications that handle time stamps typically store those time stamps in Universal Coordinated Time (UTC). The advantage of UTC is that it is universal and invariant; it is not subject to local time zones or daylight saving time (DST) changes. However, UTC is not a format that is meaningful to most users: imagine if you (like the BBC World Service does for radio programmes) referred to all events on your calendar in GMT… that might be a bit difficult to manage when talking to other people not used to GMT.


There’s a similar issue with computer software: most events with time are recorded by software and the computer in UTC, so computers convert UTC time to local time to make it easy for local users to understand. The conversion in Windows is based on two factors: first, the time zone as designated by the user; and second, whether or not DST has been selected in the Date and Time control panel. 


So, the change in DST should not in general affect the historical time stamps as recorded in UTC in documents, but it may impact how time stamps are interpreted (and therefore displayed) by the Windows operating system (folders, document properties like creation or edited times) and certain applications for a few weeks in 2007 as well as in previous years. We refer to these additional four weeks of DST (March 11, 2007 to April 1, 2007 and October 28, 2007 to November 4, 2007) as the ‘extended DST period’ – that’s the few weeks in between the second Sunday in March and the first Sunday in April. We refer to the equivalent dates in previous years as ‘previous extended DST periods.’


Revised US DST 2007


Many Microsoft and third party software applications already take this into consideration. For instance, time stamps in “track changes” and inserted comments in applications such as Microsoft Office Word, Excel and PowerPoint will record the time based on the time displayed by your Windows operating system clock. (There may some ‘home-grown’ line of business applications that don’t do this use the local system time rather than UTC.)


After you have applied the latest Windows XP time zone update, these time stamps will display the correct time for items modified during the extended DST period.  Additionally, no changes are made retroactively, so time stamps in previous extended DST periods will also display correctly. 


In Outlook, past calendar items that occurred during these previous extended DST periods may be off by an hour, as Outlook derives time based on the time zone and DST selected.


Here’s an example of the impact on displayed time stamps and what you’ll experience when you view document properties in Windows XP. Once you have applied the Windows time zone update for 2007, files created during the extended DST will display the correct ‘date modified’ time in file properties.  However, date modified time stamps that fall into previous extended DST periods (e.g., March 12, 2006 to April 2, 2006) will also be shifted by one hour. In this example, the document was created on March 20, 2006 and saved it at 7:28 PM. After you apply the Windows update, the file created last year will be erroneously updated and will show a time stamp in its ‘date modified’ properties of 8:28 PM (see below). 


Timestamps in Windows Explorer


For more info on the impact of DST, also see my reference to the article in IT Pro Magazine on daylight saving time, and Raymond Chen’s articles on “The Date/Time control panel is not a calendar” and DST for versions of Windows prior to the release of before Vista in “Why Daylight Savings Time is nonintuitive.”


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Microsoft IT webcasts on daylight saving time posted, links to upcoming DST webcasts

Our public webcast of last Tuesday on how our own IT group is handling the changes related to daylight saving time is now available for public access here.


We’re also offering a series of new webcasts over the next two weeks to help organizations preparing for daylight saving time: this is part of our “step-by-step” program on making the DST transition. Geared toward IT Professionals, we’ll walk through a general overview of DST and the impacts and solutions for Windows, Outlook and Exchange. You’ll find a list of these upcoming webcasts upcoming DST webcasts here, where we also include a few archived, on-demand webcasts.


This session is offered live on multiple dates, each beginning at 12:00 noon Pacific:



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Your questions: Where did minutes and hours originate?

A question I received this weekend: “Where did minutes and hours originate?”


Look no further than to the page on NIST’s site for general history of clocks, calendars and daylight saving time:



“A sundial described in 1300 BCE reveals that the Egyptians determined a daily cycle to be made up of ten hours of daylight from sunrise to sunset, two hours of twilight and twelve hours of night. Their calendar year was divided into 36 decans, each ten days long, plus five extra days, totaling to a 365 day year. Each decan was equivalent to a third of the zodiacal sign and was represented by a decanal constellation. The night corresponded to about twelve decans, half a day to eighteen decans. Similar to the system used in Oriental clocks, the night was thus divided into twelve hours, with seasonable variations of the hour’s length. Later, Hellenistic astronomers introduced equinoctial hours of equal length.


“The Babylonians (in about 300-100 BCE) performed astronomical calculation in the sexagesimal (base-60) system. This was extremely convenient for simplifying time division, since 60 is divisible by 2, 3, 4, 5, 6, and 10. What we now call a minute derives from the first fractional sexagesimal place; the second fractional place is the origin of the second.”


If you’re interested, I also recommend “The Experience and Perception of Time” from the Stanford Encyclopedia of Philosophy.

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Apple’s webpage for daylight saving time, Mac OS X 10.3 and 10.4 updates

I noticed that this week Apple updated their web page on daylight saving time (launched at the end of January 2007).


Apple also released Java updates for Macintosh OS X version 10.3 and version 10.4 that accomodates the latest Daylight Saving Time. Apple’s site notes that these updates include time zone information as of January 8, 2007.


Here are the links:


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Do you skip or watch commericals on a DVR?

In my last home town newspaper, the venerable Mercury News, there’s an interesting entry on the gmsv blog: “Next, we plan to send researchers to actually live among the TiVo people and study their mysterious ways” that cites this New York Times story: “People with digital video recorders like TiVo never watch commercials, right? Add that to the list of urban — and suburban — myths.”


This is something we supposed a few years ago, in that when you introduce a DVR into your home, you watch television differently. In some cases, you speed through commercials. In other situations, you may watch and replay commercials of real, direct interest, of ones that capture your attention.


“The story cites new data from the Nielsen Company showing that people with DVRs still watch, on average, two-thirds of the commercials — a reassuring finding for all stake holders in the ad business.”


That should really be a surprise to anyone with a DVR in their home… two-thirds? Really? In our home, we speed through many dull 30 second spots, but on occassion, we’ll stop and replay ones that hit the mark. But the dull certainly outnumber the ones that we’ll watch. As the article goes on to say, “Sometimes the commercials are so entertaining, you want to watch them.”


No kidding. I watch the TV to be entertained and informed — why should commercials offer anything less?


What’s the last commercial you remember watching again (‘rewind’ or back seven seconds on the old DVR)? For us, it was the trailer for the new movie “300” from Warner Brothers. It captures the dark and the frenetic nature of what promises to be a very good action film, and hopefully a good adaptation of Frank Miller’s graphic novel of the few Spartans at the Battle of Thermopylae.


That and a few commercials with anthropomorphic animals (like this one from Japan) or insurance commercials with a silly bent (I’ll admit, I enjoy some of Geico’s celebrity cameos).


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