pcdr.marty

Automatically send HTTP Header “last-modified”. 
You can use post last modification time or creation date… (settings in administration)

Last modified header

Automatically send HTTP Header "last-modified".

Evan Mattson

This plugin adds information to the admin interface about when each post/page was last modified (including custom post types!).
Enhanced areas:
<ol>
<li>Page/post admin tables – added <code>Last Modified</code> column which is also sortable.</li>
<li>Page/post edit screen (<code>post.php</code>) – added <code>Last modified on: *timestamp*</code> to <code>Publish</code> meta box.</li>
<li>Admin messages after editing a page/post – ie: <code>Post updated. *timestamp* View Post</code>,</li>
</ol>
No options currently available, but the output can be fully customized with filters and the shortcode can be easily customized using attributes!
<h3>Gutenberg, WordPress 5, and Beyond</h3>
This plugin does not yet enhance the new editor provided by Gutenberg and introduced as the default editor in WordPress 5.0. No plans exist to add support for this although it may be added in the future. 
Other areas of wp-admin enhanced by the plugin still work, as does the classic editor.

Last Modified Timestamp

Adds the last modified time to the admin interface as well as a [last-modified] shortcode to use on the front-end.

Last modified header vs Last Modified Timestamp