Every blog post should include a timestamp. In fact, this extends to almost anything online and even offline.
Also, don't make me look for the date. Put the date as obvious as possible, preferably at the beginning of the post.
I was reading Julia Evans post about nix and it was not immediately clear to me if this was a recent post. Only when I copied the URL to complain about it I discovered the date there. Safari hides most of the URL by default.