Posted by: Anthony Drewery | February 12, 2007

Changes to US Daylight Savings Time

There has been a lot of discussion recently about the changes to daylight savings time in the USA. The changes have come about due to the 2005 Energy Policy Act. In previous years, DST in most of the USA ran from the first Sunday in April until the last Sunday in October. From 2007 DST will be from the second Sunday in March to the first Sunday in November. I’m based in the UK so these changes do not affect me directly but I work for a global company with a global Windows/Exchange environment so there is an impact to address.

Microsoft has had a patch available to address the issue since late November 2006. This has recently been updated to include DST changes in other time zones around the world. My recommendation is to deploy this patch to ALL Windows machines in your environment. The patch can be distributed via WSUS if you have this facility available.

Once the patch has been deployed you may need to correct entries in the Outlook calendar. Microsoft have provided two tools to do this. The first ( is a tool that needs to be run individually for the affected users. The tool requires user interaction so you may want to distribute a link to a file share along with detailed instructions on how to run it and it’s potential impact. You should also give examples of why the tool is necessary so that the users have a good understanding.

The second tool is for Microsoft Exchange ( and will allow you to update the calendars centrally. If you are going to take this approach you need to carefully consider the risks of a blanket update. KB930879 lists the pros and cons of this method.

You can find more information from Microsoft about the DST changes here:


There are now patches for Blackberry ( and Windows Mobile (


  1. Hi,

    We are an American based company, although in London we have all our own BES and Exchange servers set to UK time zone. All our American counterparts are affected by the change in DST, but do we in London need to do anything on our systems? Will there be any effect on Americans sending us appointment notices?

    Thanks in advance!

  2. Ant,

    The OS patches work fine and do not require a reboot, the Exchange patch described in KB926666 worked fine stand alone and in a Active/Passive cluster but did requre a manual start of the Exchange services in the clustered environment. The Exchange Calendar Update Tool is not as clean as I would like an in my initial testing of over 350 mailboxes left much to be desired. Article 930879 should be used without question and read cover to cover before attempting it. It’s also my understanding that the Exchange tool is single threaded and will only process 3-4 mailboxes per minute.

    Thanks for the blog.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s


%d bloggers like this: