That's correct. All AC proofs (this includes network and log community posts and threads, as well as inbox threads) must be dated for the month the AC is for. This includes backtags. For example, for October's AC, you can only use threads and posts that were started in October; you can't use a thread from a post that was created in September even though you've been backtagging it throughout all of October. I hope this clears things up!