Ezine #117 –  DST (Daylight Saving Time)

DST – Daylight Saving TimeDST (Daylight Saving Time)

The purpose of this article is to alert you of a possible problem with computer clocks.  On March 11th this synchronization problem could affect your XP machine or your Windows 2003 server.  The significance of March 11th is that this is the date when the new DST (Daylight Saving Time) takes effect in most of America. 

Once again I thank Paul DeBrino for alerting us to the DST situation.

At the very least you may be curious to see how this computer drama unfolds on the second Sunday in March 2007.  Even if you are outside the affected zone, a one hour time discrepancy resulting from the new DST rules could affect people you deal with in America.  Incidentally, this could be a good time to review your time servers to ensure that even if DST is not a factor, your servers remain synchronised.

DST History

You may have noticed that twice a year the clocks change by one hour.  One Sunday, shortly after the vernal equinox, the clocks spring forward an hour; then just after the autumnal equinox they fall back by an hour.

Perhaps you experienced a magic moment, when you started your Windows computer on that particular Sunday and found that your computer’s clock had magically adjusted of its own accord.  Ever since the days of Windows 3.1, Microsoft has built logic into their operating system to cope with these annual time changes.

If you have any international dealings, then you may have noticed that other countries decree different days for the onset of this clock adjustment.  What you find is where there was a one hour time difference with your neighbours, it becomes two hours for a few weeks.  See when Daylight Summer Time Ends.

The Problem on March 11th 2007

The problem, particularly for Windows servers, is time skew.  Kerberos authentication fails because the clocks on the two computers are out of synch; 5 minutes is the default for Maximum tolerance for computer clock synchronization.  As ever, the system event log is great source of clues, in this instance watch out for entries with:  0x25: KRB_AP_ERR_SKEW.

What to do if think you will be affected?

I would recommend one of two extreme strategies.  Either do nothing.  Hope that all your computers are in the same boat.  Assume that all machines are all 1 hour out of synch with DST therefore all authenticate happily.  Or better – much better, research the problem thoroughly, starting with Microsoft update. http://support.microsoft.com/kb/931836/.

One possible disaster scenario is where some machines are Vista and therefore understand the new DST rules, while the other machines are XP and thus understand only the old time change rules. A variation of this problem is if some computers download the update patch but other XP machines are unable to connect with the Microsoft’s online update service.

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DST 2007 is a Problem for America (Mainly)

In 2005 the America government gave two years notice of a rule change.  The idea is that to save energy, from 2007 the clocks will spring back earlier and fall back later.  Previously the clocks sprung forward on the first Sunday in April, in 2007 DST will start three weeks earlier on Sunday March 11th.  In the autumn of 2007 the clocks won’t fall back until November 4th, one week later than previous years.

Countries affected by DST changes are: USA, Canada, Bermuda and Brazil.  Some American states may opt out e.g. Arizona and Hawaii. I have heard rumours that the Navajo Nation in Arizona will opt in.  If you are just an observer based in Europe, then be on the lookout for strange computer behaviour on Sunday March 11th.

Unlike hype about the millennium date problem, DST will be a real problem for Americans unless they take action.  As for the rest of us, we will spectators trawling the news for strange computer behaviour on Sunday March 11th.  Now that you are aware of the problem, the best thing you can do is check http://support.microsoft.com/kb/931836/

Will and Guy Humour

British Summer Time Cartoons

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