$postOrdered = array();
$queryPosts = new WP_Query(array(
    'posts_per_page' => -1,
    'post__in' => $postIds,
    'orderby' => 'usp-custom-80', // this tells WP Query to sort by distance
    'order'   => 'DESC',
    'orderby'   => 'date',
    'meta_key'  => 'usp-custom-80',
    'type' => 'DATE',
    )
);

if( $queryPosts->have_posts() ):
    while ( $queryPosts->have_posts() ) : $queryPosts->the_post();
        array_push($postOrdered, $post->ID);
        $date = usp_get_meta(false, 'usp-custom-80');
        echo $date."<br>";
    endwhile;
endif;

2-6-2015
21-12-2018
26-12-2018
18-12-2018
27-12-2018
12-11-2018
21-12-2018
7-12-2018
5-12-2018
5-12-2018
6-12-2018
19-12-2018
7-12-2018
13-12-2018
24-11-2000
25-11-2018
13-11-2018

$postOrdered = array();
$date = usp_get_meta(false, 'usp-custom-80');
$date = DateTime::createFromFormat("d.m.Y", $date)->format("m/d/Y");
$queryPosts = new WP_Query(array(
    'posts_per_page' => -1,
    'post__in' => $postIds,
    'orderby' => 'usp-custom-80', // this tells WP Query to sort by distance
    'order'   => 'DESC',
    'orderby'   => $date,
    'meta_key'  => 'usp-custom-80',
    'type' => 'DATE',
    )
);

if( $queryPosts->have_posts() ):
    while ( $queryPosts->have_posts() ) : $queryPosts->the_post();
        array_push($postOrdered, $post->ID);
        $dateOrdered =  usp_get_meta(false, 'usp-custom-80');
        echo $dateOrdered."<br>";
    endwhile;
endif;