Last Modified Column

I wanted to be able to add a Last Modified date column to my Pages and Posts page. So I came across this website, ITSupportGuides, that had a create little code snippet to accomplish just what I was looking for. Ideally this should get added to a plugin so that you can change your theme whenever you want but this code can also be placed in your themes functions.php file. Ensure you use a child theme if you do as any theme updates will replace this file.

// Add the custom column to the post type
add_filter( 'manage_pages_columns', 'itsg_add_custom_column' );
add_filter( 'manage_posts_columns', 'itsg_add_custom_column' );
function itsg_add_custom_column( $columns ) {
    $columns['modified'] = 'Last Modified';

    return $columns;
}

// Add the data to the custom column
add_action( 'manage_pages_custom_column' , 'itsg_add_custom_column_data', 10, 2 );
add_action( 'manage_posts_custom_column' , 'itsg_add_custom_column_data', 10, 2 );
function itsg_add_custom_column_data( $column, $post_id ) {
    switch ( $column ) {
        case 'modified' :
            $date_format = 'Y/m/d';
            $post = get_post( $post_id );
            echo get_the_modified_date( $date_format, $post ); // the data that is displayed in the column
            break;
    }
}

// Make the custom column sortable
add_filter( 'manage_edit-page_sortable_columns', 'itsg_add_custom_column_make_sortable' );
add_filter( 'manage_edit-post_sortable_columns', 'itsg_add_custom_column_make_sortable' );
function itsg_add_custom_column_make_sortable( $columns ) {
    $columns['modified'] = 'modified';

    return $columns;
}

// Add custom column sort request to post list page
add_action( 'load-edit.php', 'itsg_add_custom_column_sort_request' );
function itsg_add_custom_column_sort_request() {
    add_filter( 'request', 'itsg_add_custom_column_do_sortable' );
}

// Handle the custom column sorting
function itsg_add_custom_column_do_sortable( $vars ) {
    // check if sorting has been applied
    if ( isset( $vars['orderby'] ) && 'modified' == $vars['orderby'] ) {

        // apply the sorting to the post list
        $vars = array_merge(
            $vars,
            array(
                'orderby' => 'post_modified'
            )
        );
    }

    return $vars;
}