I can answer that question as I just did an experiment on this which you can review here: http://www.rlmseo.com/blog/seo-impact-blog-post-dates/

The summary is this:

Assuming no displayed date as a baseline, recent dates (not sure exactly how recent - looks like up to 3-12 months but probably depends on industry, etc) will boost traffic significantly.

Compared to the baseline, distant past publish dates will reduce traffic a good bit.

I would recommend this:

• Display update date instead of publish date.
• If a post is still relevant, update it, maybe even add a disclaimer for your users that the post was first published on x date but has been updated and is current.

We've seen dramatic traffic increases with this strategy and it ensures people know how relevant your post is, which is actually more important than the publish date.

Hi,

I'm with Philip on doing this selectively. Many users find content hard to digest in the manner it was intended when they have no idea if it was written in 2006 or 2014. If your industry is one where time really doesn't matter or change anything, this may be different. From an SEO point of view, Google still knows when it first crawled a piece of content, so not including a date won't fool Google into believing the content is fresh when it's not.

I agree that creating evergreen content where appropriate is also a good move - articles, case studies, etc. that can be updated where appropriate (and blogged about + linked to to spur re-indexing).

If you see a marked drop off in traffic to or engagement in a post from 2+ years ago that used to be successful for the site, I'd consider re-writing the post with a current slant, linking to the old one from the post and being upfront about the fact that you're refreshing an old article. You can move engagement to the new post. If there really isn't anything new to say about the subject, you can still cover it again "for new readers" and redirect the old post to the new one, canonicalise it or leave it as is if there are no duplicate content issues.

I wouldn't recommend hiding the date because you don't want users to know that the content is old.  What about when you publish something fresh and someone lands on the page but they can't find a date?  They won't know how up to date that information is.  I think a lot of people look for dates on blog posts, and rightfully so.  They want to see that they're getting good information.  You're right, if something is 2+ years old they might look for something more up to date.  But you can update old blog posts and re-date them.  Add something new to it, make some changes, and update the date.

Imagine an SEO strategy blog that didn't date the posts.  You would be doing your visitors a complete disservice by hiding the date.  You might have a post all about article directory submissions and they won't see that it's from 2008.  That's not enhancing user experience, and people won't be happy with you.

Old content won't always be a bad thing.  Read #4, "Burstiness," on this blog post: http://www.seobythesea.com/2014/03/incomplete-google-ranking-signals-1/

It's really interesting and a great read about how older content will sometimes receive the boost in rankings over fresh content.

EDIT: I'd like to add that it's completely okay to hide the date in some circumstances.  You might have some sort of evergreen content that truly will stand the test of time and info may not ever, or rarely, change on the topic.  For instance, if you were writing a blog post about how to improve your basketball shot.  Who cares if the post is from 2006?  In that case, hiding the date isn't going to reduce the overall user experience.