Last-Modified and If-Modified-Since Headers Plugin

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.

1. Upload ‘Last-Modified and If-Modified-Since Headers’ folder to the ‘plugins’ directory.
2. Activate the plugin through the ‘Plugins’ menu in WordPress.