Today was tomorrow yesterday. A sobering thought... Therefore write timeless (if not deathless) prose in web content.
People hunt for clues in content about when it was written. As you write, picture someone reading your web page tomorrow. Or next month. Will it still make sense?
All emails have dates automatically and obviously attached, and so do news stories, most blogs, press releases and Tweets. But most everyday web pages don’t.
Some web pages don’t need updating regularly, for example scholarly articles or the history of your organisation. But many are worse than useless if they are out of date.