The most recent hotfix update for the “(UTC -4:00) Santiago” time zones is available for download now. The KB2681116 was published on March 7th of 2012.
This hotfix updates Windows DST support for the [(UTC -4:00) Santiago] time zones so DST ends on April 28, 2012 at 23:59:59.999 and the clock reverts to 23:00:00.000. The update also includes the change for DST to start September 1, 2012 at 23:59:59.999 and the clock to advance to 01:00:00.000 of September 2
This update is not part of the regular cadence for publishing DST rules and Time Zone updates twice a year (fall and winter) thus it will not be available via Windows Update. Microsoft policy in response to DST and Time zone requests can be found here.
Customers in Chile that apply the update KB2681116 will be prepared for the upcoming start of the DST in Chile on April of 2012.
Hotfixes are available for support versions of Windows 7, Windows Server 2008 R2, Vista, Windows Server 2008, Windows XP and Windows Server 2003 platforms.
Based on the information published by Chilean government here the start of the DST period in 2012 will follow the rules below.
Start of the DST Period:
For the mainland and Chilean Antarctica the Daylight saving time ends:
On April 28th, 2012 at 23:59:59
For the mainland and Chilean Antarctica the Daylight saving time Starts:
On September 1st, 2012 at 23:59:59
You can request the update by visiting KB 2681116 or directly here, once you are in the hotfix request site make sure to select the option "Show hotfixes for all platforms and languages" to view the complete list of languages and operating systems.
Note: Windows 2008 and Windows Vista update packages are built from the same servicing branch and are shown under the Vista Product name on the download page. The Vista titled package does install and update on Windows 2008.
For more information see Microsoft Knowledge Base: KB2681116 Daylight saving time (DST) update for Chile.