Unfortunately with the Direct Results, we pass your search query straight through to craigslist, so we can only support the options that they do. To the best of my knowledge the only date filter they offer is the "today's results" one, so we're limited to that as well.
One thing you can do that might help is make sure you have it set to sort listings by date, then whenever you search, click on the first result in each list, whatever it is. Then next time you're searching, that one will be shown as visited (purple or whatever), so you'll know you've seen the ones below it. Kind of a hack, but could be useful.