Dogan Ucar

WP Last Modified adds the ‚last_modified_date‘ shortcode to your WordPress installation. This shortcode shows the last timestamp of your post/page. Simply use the „format“ attribute for custom date formats (it uses PHP’s date() function). 
The “description” attribute enables a brief description which changes has been made with the last modification. 
WP Last Modified shows also the actual revision of and the publication date of the post/page. 
You can customize the text which will be shown under each post/page under settings -> WP Last Modified Settings. You can also hide one of the elements if you do not want them.
With version 1.1.0, WP Open Last Modified was renamed to WP Last Modified.
Features:
<ul>
<li>last modification timestamp of a post/page</li>
<li>publication timestamp of a post/page</li>
<li>timestamp format customizable</li>
<li>revision count</li>
<li>custom text with placeholders</li>
<li>last modification description</li>
<li>excluding single posts/pages from displaying the last modification timestamp</li>
</ul>

WP Last Modified

This plugins adds the last modified date, current revision and the publication date of your post/page. Simply use the shortcode [last_modified_date]

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".

WP Last Modified vs Last modified header