If Modified Since HTTP Header
the_modified_date() is a template tag that must used inside the loop, that is why it is not wokring for you. WordPress provide a action and filter hook to include or modify HTTP headers: send_headers action wp_headers filter But it doesn’t work for this purpose. For example, the next code is not working: add_action( ‘send_headers’, ‘cyb_add_last_modified_header’ … Read more