According to
this article, the change was approved on Dec 28, and implemented today. That's just 2 days from approval to implementation
Argentine Congress OKs Time Change to Save Energy
ARGENTINA: December 28, 2007
BUENOS AIRES - Argentina's Congress approved a daylight savings measure late on Wednesday, part of a broader government plan to conserve energy as demand for power surges amid brisk economic growth.
Under the new law, which both houses of Congress passed the same day, the time in Argentina will jump forward an hour starting on Sunday. The clocks will turn back on March 16.