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]

marco.rucci

This plugin allows you to create strong evidence of intellectual property on the post content by certifying a blog post using the criptographically secure process of trusted timestamping. The timestamps are issued by an accredited Certification Authority, thus adding legal value to the certification. The certification guarantees that the post content is not modified since it has been published.
Coming soon: each certified post will show a “certification seal” that links to a page on easytimestamping.com where verification is performed on the fly. The verification checks that the post content has not been altered and checks that the timestamp associated with the post has been issued by a Certification Authority and verify its authenticity. The “certification seal” allows the viewers of your blog to verify that the content of the blog post has never been altered since its certification date.

Certify Post

Create strong evidence of intellectual property of your posts and show it to your viewers.

WP Last Modified vs Certify Post