Bill Erickson

When editing a post, check “Mark as Old” in the metabox labeled “Old Post Notification” to add the notification to the top of the post. You can also define a custom message for this specific post. If no custom message is provided, the default message set in Settings > Old Post Notification will be used.
Further Customization with filters, described in documentation.
<a href="https://github.com/billerickson/old-post-notification/wiki" rel="nofollow ugc">Documentation</a> | <a href="https://github.com/billerickson/old-post-notification/issues" rel="nofollow ugc">Support Forum</a>

Old Post Notification

Mark posts as old and display a notification message above the content.

Mario Peshev

DevriX

DX Out of Date allows you to display a notification box on your posts 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or any content that might get outdated due to external factors – now you 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:
<ul>
<li>Period – “amount” of “duration” units (1 to 40)</li>
<li>Duration frame (days, months, years)</li>
<li>Message – the message format for the notification box</li>
<li>Enable by default – a checkbox for enabling the message on all posts by default</li>
<li>Enable post status in Dashboard – a checkbox for adding an extra column in the Dashboard for the outdated status of posts</li>
<li>Post types to affect – allows you to choose which post types for display the message for</li>
<li>Position of the message – choose whether the message should be displayed at the top or at the bottom of a post’s content</li>
<li>Background color – Choose a background color for the message</li>
<li>Text color – Choose a text color for the message</li>
<li>Custom CSS – Add custom CSS rules to the message box</li>
</ul>

DX Out of Date

Display a notification box on the single view of your outdated posts, given a set amount of time (configurable).

Old Post Notification vs DX Out of Date