by Mario Peshev on WordPress.org
Display a notification box on the single view of your outdated posts, given a set amount of time (configurable).

DX Out of Date Settings page.
DX Out of Date allows you to display a notification box on your posts
when a given amount of time has passed. Quite handy when writing tutorials
or any content that might get outdated due to external factors – now you
could automatically notify your readers after a given period.
You can configure the expiration time of your posts in the admin panel. Valid time units are days, months and years.
Currently the plugin options are: