If it is a great post, why does it matter when it was written? How to win friends and influence people was written close to 100 years ago, but it is still reco

If it is a great post, why does it matter when it was written? How to win friends and influence people was written close to 100 years ago, but it is still recommended in plenty of places. The dragon book is older than me, but it is still one of the recommended books to write a compiler in. Why does a blog post have to have a date displayed on it? If it is about a specific version of some software, I can understand, and agree with you, why it should be mentioned in the post.