[WordPress] 初回の記事公開日をカスタムフィールドに残す via. https://qiita.com/gatespace/items/a8853c96f892426bfabe

manage_posts_columns.php

add_filter( 'manage_posts_columns' , 'add_publish_date_columns' );
function add_publish_date_columns( $columns ) {

    $new_columns = array();
    foreach ( $columns as $column_name => $column_display_name ) {
        if ( $column_name == 'date' ) {
            $new_columns['publish_date'] = '掲載日時';
        }
        $new_columns[ $column_name ] = $column_display_name;
    }   
    return $new_columns;
}
add_action( 'manage_posts_custom_column' , 'custom_publish_date_column', 10, 2 );
function custom_publish_date_column( $column, $post_id ) {
    switch ( $column ) {
        case 'publish_date' :
            if ( ! empty( get_post_meta( $post_id, '_publish_date', true ) ) ) {
                echo date_i18n( get_option('date_format') . get_option('time_format'), get_post_meta( $post_id, '_publish_date', true ) ); 
            }
            break;

    }
}

save_post.php

add_action( 'save_post', 'my_save_post' );
function my_save_post( $post_id ) {
    if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) {
        return;
    }
    if ( wp_doing_cron() ) {
        return;
    }

    if ( get_post_status ( $post_id ) == 'publish' || get_post_status ( $post_id ) == 'future' || get_post_status ( $post_id ) == 'private' ) {
        if ( empty( get_post_meta( $post_id, '_publish_date', true ) ) ) {
            $publish_date_u = get_the_date( 'U', $post_id );
            update_post_meta( $post_id, '_publish_date', $publish_date_u );
        }
    }
}