如何在WordPress中基于月份和日期对循环进行排序？

Question

I have a loop that returns a list of birthdays. Currently, I'm using orderby => meta_value_num sort and it sorts year as well. See the code below:

            $today = date('d');
            $tomorrow = date('d', strtotime('+ 1 day'));
            $thedayaftertomorrow = date('d', strtotime('+ 2 day'));
            $current_month = date('m');
            $next_month = date('m', strtotime('+ 1 month'));
            $args = array (
              'posts_per_page' => 4,  // number of posts   
              'post_type'      => 'employees', // your custom post type
              'meta_key'       => 'birthday',  // your custom date field name
              'orderby'        => 'meta_value_num',
              'order'          => 'ASC',
              'meta_query' => array(
                'relation' => 'OR',
                array(
                  'key'      => 'birthday',
                  'compare'  => 'REGEXP',
                  'value'    => '[0-9]{4}' . $current_month . $today,
                ),
                array(
                  'key'      => 'birthday',
                  'compare'  => 'REGEXP',
                  'value'    => '[0-9]{4}' . $current_month . $tomorrow,
                ),
                array(
                  'key'      => 'birthday',
                  'compare'  => 'REGEXP',
                  'value'    => '[0-9]{4}' . $current_month . $thedayaftertomorrow,
                ), 
                array(
                  'key'      => 'birthday',
                  'compare'  => 'REGEXP',
                  'value'    => '[0-9]{4}' . $next_month . '[0-9]{2}',
                ),
              )
            );

Is there a way to ignore year and order by month and day? Date format I'm using is: Ymd

The code returns the following:

1990-12-01 1992-01-04 2001-03-04 2005-06-07