PHP array change value

one text

I'm stuck at a small problem, I tried some things but it didn't work. I have a reservation system (in Wordpress) that generates a database with all info. Everything works fine except the sorting of the reservations on date. It sorts on day and month but ignores the year. The reason: format of the date is a string = Month/Day/Year (ex. 05/20/2021). This can't be changed in the database!

The solution would be to change all values in the array to a real date or just a numeric value like YYYYMMDD (ex. 20210520). But for some reason I can't get it right. Can you help me?

This is my array:

$args = array(
    'post_type' =>  'homey_reservation',
    'meta_key' => 'reservation_checkin_date',
    'orderby' => 'meta_value',
    'order' => 'ASC',
    'paged'             => $paged,
    'posts_per_page'    => $listing_no,
);

foreach ($args as $meta_key => $field) {
    $split = explode("/",$meta_key);
    $day = $split[1];
    $month = $split[0];
    $year = $split[2];
    $field = $year.$month.$day;
}