Beginning Winter Time:

a. - In the Continent and Chilean Antarctic

At 24:00 on Saturday April 28, 2012, set  back one hour becoming 23:00 of the same day.   
(GMT/UTC - 04)

b. - Easter Island and Salas y Gómez Island

At 22:00 on Saturday April 28, 2011, set back one hour becoming 21:00 of the same day.  
(GMT/UTC - 06)