There are two directions people go with their blogs – having dates with their posts and not having dates. I vote yes for putting the date with your post. Reasons?
1. It shows your readers that you are staying current with your blog.
2. It gives your readers a frame of reference for when the material was created. This is important for determining if the information might be relevant. In many fields these days, the latest information is vital. (This is true for social media, the area I work in.)
The other argument is that “my information is timeless”. That may be true – but your readers would like to know when you thought that particular thought. Maybe you have changed your thinking since then! (Not many people take time to go back to update all of their posts.)
If you are giving timeless information – and it’s valuable – just post it again, in a different way. Readers are smart enough to search your blog for that topic, if they liked what you had to say.
Finally, sometimes I actually will leave a blog when the blog is giving technical information and there is not date with the post.
Most blogs that I read have time stamps.
Seems like it would be good to put them inside text, too, so that when people forward “news” by e-mail, it becomes clear when it is out of date.
If there is “timeless” stuff, they can post shortcuts on their front page or refresh the time stamp from time to time.
I vote “yes, include the date”. I don’t know how many times I’ve gone to a blog to see the writer’s opinion of something — often a product — and wished for a date to help me determine whether his/her opinion was current or ‘old news’.