The U.S. Energy Policy Act of 2005, passed by the U.S. Congress July, 2005, extended Daylight Saving Time (DST) in the U.S. by approximately four weeks. As a result, beginning in 2007, DST will start three weeks earlier on March 11, 2007, and end one week later on November 4, 2007, resulting in a new DST period that is four weeks longer than previously observed. These four weeks are referred to as the "extended DST period".

Previously DST started on: First Sunday of April (04/01/07) 
New DST will now start this year on: Second Sunday on March (03/11/07)

Previous DST ended on: Last Sunday of October (10/28/2007)
New DST will now end on: First Sunday of November (11/04/07)

Unless certain updates are applied to computers, it is likely that the time zone settings will be incorrect during this four week period. This will of course depends on where you live and which time zone you have selected.

If your time zone settings are incorrect your clock may be off by one hour, and certain applications running on your Windows based computer may not display the correct time.  I don't expect this change in DST to cause serious havoc to most users but it will affect applications that rely on clock settings such as Time and Billing Software, Payroll Applications, Calendaring and Scheduling Software, CRM, etc...

Microsoft and other vendors have issued a series of patches to address this problems and you will find ton of information on the web.  Try to google "DST 2007" and you will see what I mean.  I will save you time with Microsoft and a good place to start is: http://support.microsoft.com/gp/dst_topissues#A 

Some other useful links as well:

Microsoft Windows OS for Servers: Servers:  http://support.microsoft.com/kb/931836/en-us

Microsoft Windows OS for PC's:  http://support.microsoft.com/kb/931836/en-us

Microsoft Exchange 2003 (If you use Exchange 2003)http://support.microsoft.com/kb/926666/en-us 4

Microsoft Exchange Calendar Update Tool: http://support.microsoft.com/kb/930879/en-us 5.      

Microsoft Outlook (All Versions included): http://support.microsoft.com/kb/931667/en-us 6.      

Microsoft Sharepoint Services 2.0 (If you use Sharepoint): http://support.microsoft.com/kb/924881/en-us

Microsoft CRM 3.0: http://support.microsoft.com/kb/925874  

In addition to Microsoft, please refer to the following list for 3rd party software or hardware that may be affected by the DST     

BlackBerry:  http://www.blackberry.com/select/dst2007/index.shtml   

Symantec:  Symantec Support
    
Novell: http://www.novell.com/info/daylight_saving.