A current workaround is to just isolate your search by pagination. Enter the rest of your search normally and then choose to sort by Newest. If you're really good, you can pick out the proper page off-hand and just enter it straight into the URL (add "page=X" to your query string, "pagesize=Y" as well if necessary). Otherwise, I'd start picking random pages based on how recent or old the target date is. Once you find a post from that date, just work backwards or forwards as you need.

This naturally doesn't work with any other sort order (but personally I've never found "relevant" to be as useful as "newest" or "active" have been), and it doesn't really change the number of results. But it can help for some points, at least for now.

Ironically, the times that I have actually used this workaround (which is twice), the target question I want was actually deleted so I never found anything.