header-last-modified to the /wp-content/plugins/ directoryheader_last_modified() in /your_website_path/wp-includes/template-loader.php before "if ( $template = apply_filters( 'template_include', $template ) )" .
Like this:
header_last_modified();
if ( $template = apply_filters( 'template_include', $template ) )
include( $template );
return;