Get current user last post date

Is that the only code you’ve used? As the name suggests that function returns the entire post, not just the date. You still need to actually get the date from that post.

$user_id = 2;
$post = get_most_recent_post_of_user( $user_id );

if ( $post ) {
    echo get_the_date( 'Y-m-d', $post );
}

See the PHP documentation on date formats for other ways to format the date.