It's 365 days back from the date of your most recently rated round. If your total number of rounds in that period is fewer than 8, then we go back farther to get 8 or whatever you have in 24 months if still less than 8.
But since my most recently rated rounds were from May 29th, 2006, that round from last June will still be included until the September update. Is that correct? Thanks, Chuck.