Retrieve post modified date for specific post by post ID

Place this code snippet put in function.php, this should give you what you need.

function my_theme_wp_title( $title, $sep ) {
    global $post;

    /* my other title cases */

    //get post's modified date
    $m_date = get_the_modified_date();

    //concatenate the current title with the date string, using the separator to be more clear
    $title .= $sep . " " . $m_date;

    return $title;
}
add_filter( 'wp_title', 'my_theme_wp_title', 10, 2 );

Basically we add our custom function hooked to the wp_title filter, thus altering wp_title function, giving us the flexibility to add custom logic in the title display.

Cheers!