Joystiq reports that the new end date is Sunday, June 23, at midnight PT. This weekend will also offer double XP in your auto-matched modes, so you can grind out some experience.
As previously reported, your experience will transfer to the main game as long as you buy the full version within 60 days of launch and tie it to the same account. That means your hard work this weekend won't be wiped when the game launches next Tuesday, and you get a chance at a head start. Any advantage is a good advantage when taking on the Russians, boy.