zubovd

This plugin adds the ‘Last-Modified’ header to each post and returns the ‘304 Not Modified’ header to the client without a body in the response if he sent the ‘If-Modified-Since’ header and the post has not changed since that date.

Last-Modified and If-Modified-Since Headers

Add Last-Modified anв support If-Modified-Since Headers

ezraverheijen

If Modified Since tells search engine spiders one of two things about a webpage:
<ul>
<li>This webpage has not changed, no need to download again.</li>
<li>This webpage has changed so download again because there is new information.</li>
</ul>
If the requested page has not been modified since the time specified by a search engine spider, a 304 (not modified) response will be returned without any message-body.
If you have any issues using If Modified Since, find a bug or have an idea to make the plugin even better then please <a href="https://github.com/ezraverheijen/wp-if-modified-since" rel="nofollow ugc">help to improve If Modified Since</a>. 
If you don’t report it, I can’t fix it!

If Modified Since

Comply with Webmaster Guidelines, save some bandwith and reduce server load with the If-Modified-Since header.

Last-Modified and If-Modified-Since Headers vs If Modified Since