It's based on calendar date. So if you have purchases that occurred in December 2008 and posted in December 2008, it applies to 2008 regardless of whether it is on your December or January statement.

The confusion comes into play with transactions that occur in late December but post in January. I often have purchases on my January statement that list a transaction date of 12/31 for example but a posting date of 1/2 or later. I should find out where those count later in the year as it was over $1500 for me that falls into that category.