If you do the Plat Challenge after 16 June, you keep AA Plat until the end of Feb in 2 years time. i.e. start 16 June 2005 and get Plat until 28 Feb 2007.
Start BEFORE 16 June and you only get Plat until 28 Feb 2006. (Feb is the cut-off point for AA membership years, as I understand it.)