archive status

Possibly I missed this somewhere, but does Sandy automatically categorize some things as archived in her database based on some internal date rules? The reason I ask is that I have some items tagged for a daily log I keep, and when I click on the tag, only the items from this month show up. When I click on the "include archive" link on the right, the one's from last month now show up. While they are tagged @done, I never tagged them @archive, nor do they show that in their respective tags.