Sort posts on max date in Custom Field

   // List custom posts in ascending order of the maximum date
   // contained in a list of dates in a custom field.  The dates
   // are in d-m-yyyy format.
   // Example meta_value: 1-10-2011, 17-2-2013, 25-6-2014

   $post_type = 'parks';
   $meta_key = 'dates_of_leaving';

   // Get post IDs and dates_of_leaving
   $sql = "
      SELECT p.ID, pm.meta_value AS dates
      FROM $wpdb->posts p
      JOIN $wpdb->postmeta pm ON (p.ID = pm.post_id)
      WHERE p.post_type = '$post_type'
         AND pm.meta_key = '$meta_key'
         AND p.post_status = 'publish'
      ORDER BY p.ID ASC
   ";
   $rows = $wpdb->get_results($sql);

   // Get the max date for each ID. Save the ID and date for sorting
   $for_sorting = array();
   $current_id = 0;
   foreach ($rows as $row) {
      $newdates = array();
      $olddates = explode(',',$row->dates);
      // Reformat and save the dates
      foreach ($olddates as $olddate) {
         if (preg_match('/(\d+)-(\d+)-(\d{4})/', $olddate, $matches ) ) {
            $newdates[] = $matches[3] .
            str_pad($matches[2], 2, '0', STR_PAD_LEFT) .
            str_pad($matches[1], 2, '0', STR_PAD_LEFT);
         }
      }
      rsort($newdates);  // Sort the dates high to low
      //print_r('<pre>DATES:');print_r($newdates);print_r('</p>');
      if ($newdates) {
         $for_sorting[] = array('ID' => $row->ID, 'date' => $newdates[0]);
      }
   }
   //print_r('<pre>FORSORTING:');print_r($for_sorting);print_r('</p>');

   // Now, sort the array with ID and date
   usort($for_sorting, function($a, $b) {
        return $a['date'] - $b['date'];});
   //print_r('<pre>SORTED:');print_r($for_sorting);print_r('</p>');

   // Get just the IDs, in order
   $id_array = array();
   foreach ($for_sorting as $sorted) {
      $id_array[] = $sorted['ID'];
   }
   // Show posts in order of post__in array
   $args = array(
      'post_type' => $post_type,
      'post__in' => $id_array,
      'ignore_sticky_posts' => 1,  // Stickies will mess this up
   );
   query_posts($args);
   if (have_posts()) :
      usort($wp_query->posts, function($a, $b) use ($id_array) {
         return array_search($a->ID, $id_array) - array_search($b->ID, $id_array);
      });
      while (have_posts()) :
         // Put your own loop code here
         the_post();
         echo "<p>ID: $post->ID</p>";
      endwhile;
   endif;