Inform user about automatic comment closing time

To prevent spammers from flooding old articles with useless comments you can set WordPress to close comments after a certain number of days:

It might be surprising for some users if the comments are closed automatically so it might be a good idea to inform them about the remaining time.

add_action( 'comment_form_top', 'topic_closes_in' );

function topic_closes_in() {
    global $post;
    if ($post->comment_status == 'open') {
        $close_comments_days_old = get_option( 'close_comments_days_old' );
        $expires = strtotime( "{$post->post_date_gmt} GMT" ) +  $close_comments_days_old * DAY_IN_SECONDS;
        printf( __( '(This topic will automatically close in %s. )', 'domain' ),  human_time_diff( $expires ));
    }
}

While the code should be almost self explanatory there is an interesting function not every WordPress developer might know: human_time_diff(). This function is hidden in the .../parts/formatting.php file. It is originally planned to be used in themes to provide a more “human” touch to the date/time a post was written. Since the function does not care if the date is in the past or in the future we can use it for our needs.