Add Last Modified Date To Post Columns

<?php
class Add_Last_Modified_Date_To_Post_Columns {

    function __construct() {
        add_filter( 'manage_posts_columns',              array( $this, 'add_column' ), 10, 2 );
        add_filter( 'manage_edit-post_sortable_columns', array( $this, 'add_sortable_column' ), 10, 2 );
        add_action( 'manage_posts_custom_column',        array( $this, 'display_column' ), 10, 2 );
        add_action( 'admin_enqueue_scripts',             array( $this, 'add_inline_style' ) );
    }

    function add_column( $posts_columns, $post_type ) {
        $posts_columns['modified'] = __( 'Last Modified' );

        return $posts_columns;
    }

    function add_sortable_column( $sortable_columns ) {
        $sortable_columns['modified'] = array( 'modified', true );

        return $sortable_columns;
    }

    function display_column( $column_name, $post_id ) {
        global $mode;

        if ( 'modified' !== $column_name ) {
            return;
        }

        $post = get_post( $post_id );

        if ( '0000-00-00 00:00:00' == $post->post_date ) {
            $t_time = $h_time = __( 'Unpublished' );
        } else {
            $t_time = get_the_modified_time( __( 'Y/m/d g:i:s A' ) );
            $m_time = $post->post_modified;
            $time = get_post_modified_time( 'G', true, $post );

            $time_diff = time() - $time;

            if ( $time_diff > 0 && $time_diff < DAY_IN_SECONDS ) {
                $h_time = sprintf( __( '%s ago' ), human_time_diff( $time ) );
            } else {
                $h_time = mysql2date( __( 'Y/m/d' ), $m_time );
            }
        }

        if ( 'excerpt' === $mode ) {
            echo $t_time;
        } else {
            echo '<abbr title="' . $t_time . '">' . $h_time . '</abbr>';
        }
    }

    function add_inline_style( $hook_suffix ) {
        if ( 'edit.php' !== $hook_suffix ) {
            return;
        }

        wp_add_inline_style( 'wp-admin', '.fixed .column-modified { width: 10% }' );
    }

}

new Add_Last_Modified_Date_To_Post_Columns;
?>