It automatically archives every thread that's 31 days old...
so every day it adds a few to the archives (those that are a month old). The archive creates a new date range to choose from every 250 threads. Now it has started it's second date range, and we're filling that one up each day with the 31-day old posts, until it hits 250 threads, and then it will create a third date range. Hope that makes sense. I had to ask Dan how it was working too.
I told Dan I would like to be able to create a new date range on the first of every month, or something like that, and he's going to try to build that option into the next update.