I recently read a post on Elegant Themes (aff link) that suggests you should remove the dates from your blog comments and “…Keep Your Discussion Fresh.” They proceed to show you multiple ways to remove dates from posts and comments. Don't do it.
Dates on blog posts are critical for signaling relevance and context to your reader. I can't count the number of times I've read a post I thought was relevant only to realize midway through (because the dates were scrubbed) that it was two or three years old and a pointless waste of time. Now, to be fair, some posts really are date-independent—the information is truly timeless and when it was posted is not important. But in cases like this, I would argue that this material should be reserved for an article section that is built on pages, not posts, and then dates can be removed without trouble.
As far as comments, it's also critical to know the dates, especially in those cases where comments are offering suggestions for alternative materials/software/ideas that are time sensitive. (Many posts I read and comment threads I follow are about X software for Y purposes.) You can save your reader a lot of wasted time and effort by signaling right up front that a comment, or entire thread, is too old to be relevant. And if that's the case, now you have a reason to write a new one!
This date-removal strategy is one that only serves the publisher and not the reader. If you really wish to help your reader, you'll aid them in every way possible. To that end, signaling relevance via clearly posted dates and references is critically important—it helps the reader place your ideas within a specific context. For blogs and publications that feature non-fiction and reviews and how-tos, this is even more important and helps readers quickly understand if they're in the right place at the right time.
While possibly less important for fiction and stories, keeping dates still allows for understanding where one is and reduces confusion in general which, in turn, creates a relaxed state of mind and more favorable perception of the work. It's really inspiring to see a story progress over time and understand how the author's style changes. This is hard to do when it all happened “right now” because dates are missing.
Having your time-sensitive material slowly become less relevant over time is a reality that comes with the blogging experience and it's better to accept that and keep posting new and better material rather than trying to hoodwink your reader into wasting time on old work that might not be relevant anymore.
I would argue that having your posts age out is actually a great thing. By looking at the old material that might be garnering more attention, you can know that the readers are interested, despite the date, and now have material for an excellent follow-up post that revisits the original subject and adds additional info and clarifications. You can then link to the old post and start new, current discussions, possibly on both the old and new posts. This can also boost the relevance of the old and new posts in Google's eyes.
I think there's too much focus on so-called “evergreen” material and the author/publisher's needs and not enough focus on remembering who we're writing for in the first place. Your reader is your number one, primary focus!
Instead of trying to pass old and busted off as the new hotness, go the extra mile and publish more and better work with all the dates and references in place so we can better understand the context. Your readers will love you for it and reward your efforts in kind.
As for me, when I see a blog that has hidden its dates, I immediately think the author is trying to scam me (and also thinks I'm an idiot who won't notice). And you can imagine how well that works as a first impression.
[All ideas and replies welcome.]