Automatically update blog post date/time?

Old Post Promoter is another good one, in answer to the OP's question. It doesn't do what you're requesting, but it will update the dates and things on old posts in your feed, which is REALLY what you want.

But in response to this question, it shouldn't as long as you are using a permalink structure that is not date sensitive (this would mean you're creating multiple URLs with the same content every time you update the date. This (unlike nearly everything else people say about "duplicate content") could ACTUALLY cause you some ranking issues.

However, if your permalinks do not contain dates, using a plugin like this can be VERY beneficial if you know what you're doing. First, if you are doing this on a blog without ever producing new content, TURN OFF the pinging. Pinging pages that don't change multiple times will get your pinging account suspended. Only ping pages that CHANGE.

Second, once you have your pings off, permalinks set, and plugin in place - take your RSS feed and submit it to as many places as possible.

What will happen with a plugin like OPP and a clean dateless theme is that you will appear to have a non-dated article-centric mini site. The site will appear largely static. But because of your constantly rotating RSS feed, you will get continuous traffic and backlinks JUST AS IF you were constantly updating your blog's RSS.

Anyway, that's something I've experimented with in the past with GREAT success, so I think it will work well for you too!

Note: you don't NEED to do this with a dateless mini-site theme. That's just the way I did it. It should work okay with a normal dated blog, it just looks funny to me.