For the year 2009/2010, summer time in Brazil by the Brazilian government is determined as follows:
In the past year Microsoft has updated DST rules for Brazilian time zone - (GMT -3:00) and Brasilia (GMT -4:00) Manaus, in the Windows operating system, with the following parameters:
In 2009, the day after the "Second Saturday in February" corresponded to the "Third Sunday in February," therefore, the rule for the time zone in Windows operating system corresponded with the dates established by the Brazilian government:
However, in 2010, the day after the "Second Saturday in February" is the "Second Sunday of February." This means that the time zone currently defined in the Windows operating system is behind the dates set by the government Basileiro in a week.
Because of the difference explained above, the Windows operating system does not interpret correctly the time related to the end of daylight saving time in Brazil in 2010 (21 February 00:00).
Additionally, the week of "14 - February 20, 2010", the Exchange and Outlook users may experience discrepancies in the calendar - once the operating system is set, appointments created during this week may be delayed by one hour.
RECOMMENDATIONS FOR WINDOWS USERS - HOME OF THE BRAZILIAN SUMMER TIME
There are no special instructions for transitioning to the start of Daylight Saving Time (October 18, 2009) for the following scenarios:
Machines that are updated with KB 955839 and KB 970653 available through the "Windows Update":
Machines adjusted according to the instructions published last year in the blog Latam .
ADDITIONAL INFORMATION - END OF SUMMER TIME BRAZIL
Users must follow additional instructions that are prepared to transition to the end of daylight saving time on February 21, 2010. Microsoft provides for December 2009, the launch of a new cumulative update for timezone (time zone), containing the necessary changes to the adjustment related to the end of Daylight Saving Dollars. When this cumulative update for timezone (time zone) is released, will also be available guidelines / instructions for Windows, Exchange and Outlook.
Source: http://blogs.technet.com/latam/archive/2009/09/08/brazil-dst-2009-2010.aspx