American Express Membership Rewards - Billing Cycle Question

I have a lot of trips to purchase for early next year, and I want to put them on my Hilton Amex card to meet my $20000 spend for the 2009 calendar year. So, when can I start buying these tickets to get the charges on the 2009 billing year? Do I have to wait until 1/1/09? Any help will be appreciated. Thanks
Radonc1

the 20k per calendar year are not coupled to your billing cycle. The new 2k qualification period will start with charges from 1/1 onwards.