wordpress get_posts ordered by custom field which is a custom date

Question

i have a custom post type for members and the birth date is saved like 01.01.2013

$args = array(
    'numberposts'   => -1,
    'post_type'       => 'mitglied',
    'post_status'     => 'publish',
    'meta_key'      => 'geburtsdatum',
    'meta_value'    => date("d.m.Y"),
    'meta_compare' => '>',
    'orderby' => 'meta_value',
    'order' => 'ASC'
);
$geburtstage = get_posts ( $args );

i want a list with the birthdays of the members ordered by month and day like this

• 4th of january, person 10
• 8th of march, person 2
• ...
• 1st of december, person 3

with the code above the list is ordered by day

• 1st of december, person 3
• 4th of january, person 10
• 8th of march, person 2

also important is, that the ordering is not influenced by the year of birth

Answer 1

I think my task is not solveable with get_posts only, so i did it in 3 steps.

First, i used get_posts to get all members

$args = array(
    'numberposts'   => -1,
    'post_type'       => 'mitglied',
);
$geburtstage = get_posts ( $args );

Then filled an array with the data. For sorting I made a timestamp with the day and month of the birthday, but the actual year. Although with this solution it would not have been necessary, I changed the date format to yyyy-mm-dd. @barakadam explained in his answer to my question how to do this. I use the wordpress plugin Advanced Custom Fields, so I fetch the custom fields with get_field()

$mitglieder = array( array() );
$i = 0;
foreach($geburtstage as $post){
    setup_postdata( $post );

    $datum = explode( "-",get_field('geburtsdatum') );

    $mitglieder[$i]['orderby'] = mktime(0, 0, 0, (int)$datum[1], (int)$datum[2], date('Y'));
    $mitglieder[$i]['name'] = get_field( 'name' );
    $i++;
}

The last step is using array_multisort to get the intended order

foreach ($mitglieder as $key => $row) {
    $dates[$key] = $row['orderby'];
}
array_multisort($dates, SORT_ASC, $mitglieder);

Thanks to @barakadam and @Christopher for participating.