The beta search does sort based on OP posting date, just not exclusively. Results favor newer topics over older ones. When ranked based on the keyword query, results are penalized for being old thus with many posts being equally good matches on keyword, say 2000 as a "score", 1 point is then deducted for each day old the topic is.

So a topic from today would have X points, an identically good match posted a year ago would have X-365 points.

I realize that isn't what people want necessarily but it's the simplest way to do "simple" search with reasonable results across all the topics and posts. "Advanced" searching (filtering and sorting) hasn't been finished yet for the beta search engine.