Order by custom field date?

I’ve created a category “Expire Soon” and i want to insert and sort the posts by custom field “expiration_date”.

I use this code but the posts are sorted by custom field “expiration_date” but not by the date. The posts are sorted by the numbers in the date. Such as:

31/12/2012
28/01/2013
15/01/2013
04/12/2012

and i want to be like:

04/12/2012
31/12/2012
15/01/2013
28/01/2013

The code i use to insert the custom fields (creating them with Advanced Custom Fields) is:

<?php
                                            if( get_field( "dwra_diagwnismou" ) ): ?>
                                                    <p><b>Δώρα διαγωνισμού:</b>   <?php the_field( "dwra_diagwnismou" ); ?></p>
                                            <?php endif;

                                            if( get_field( "kathgoria-diagwnismou" ) ): ?>
                                                    <p><b>Κατηγορία διαγωνισμού:</b>   <?php the_field( "kathgoria-diagwnismou" ); ?></p>
                                            <?php endif;

                    if( get_field( "diorganwths_diagwnismou" ) ): ?>
                                                    <p><b>Διοργανωτής διαγωνισμού:</b>   <?php the_field( "diorganwths_diagwnismou" ); ?></p>
                                            <?php endif;

                    if( get_field( "hmeromhnia_lhkshs" ) ): ?>
                                                    <p><b>Ημερομηνία λήξης:</b>   <?php the_field( "hmeromhnia_lhkshs" ); ?></p>
                                            <?php endif;

                    if( get_field( "wra_lhkshs" ) ): ?>
                                                    <p><b>Ώρα λήξης:</b>   <?php the_field( "wra_lhkshs" ); ?></p>
                                            <?php endif;

                    if( get_field( "periorismos_perioxhs" ) ): ?>
                                                    <p><b>Περιορισμός περιοχής:</b>   <?php the_field( "periorismos_perioxhs" ); ?></p>
                                            <?php endif;

                    if( get_field( "apaitei_logariasmo_facebook" ) ): ?>
                                                    <p><b>Απαιτεί λογαριασμό Facebook:</b>   <?php the_field( "apaitei_logariasmo_facebook" ); ?></p>
                                            <?php endif;

                    if( get_field( "syxnothta_diagwnismou" ) ): ?>
                                                    <p><b>Συχνότητα διαγωνισμού:</b>   <?php the_field( "syxnothta_diagwnismou" ); ?></p>
                                            <?php endif;

                    if( get_field( "link_diagwnismou" ) ): ?>
                                                    <p><b>Link διαγωνισμού:</b>   <a href="<?php the_field( "link_diagwnismou" ); ?>"><?php the_field( "link_diagwnismou" ); ?></a></p>
                                            <?php endif;

                    if( get_field( "oroi_diagwnismou" ) ): ?>
                                                    <p><b>Όροι διαγωνισμού:</b>   <?php the_field( "oroi_diagwnismou" ); ?></p>
                                            <?php endif;

                    if( get_field( "plhrofories_diagwnismou" ) ): ?>
                                                    <p><b>Πληροφορίες διαγωνισμού:</b>   <?php the_field( "plhrofories_diagwnismou" ); ?></p>
                                            <?php endif;

                                         ?>

I use it in the loop.php

If anyone can help me sort by date i would be grateful!

@s_ha_dum
This is the code i use in the functions.php to insert the data from the submitted form to the custom fields meta-box in the post edit:

/* ------------------------------------------------
   QuForm Plugin - Form to Post
------------------------------------------------ */

add_action('iphorm_post_process_1', 'mytheme_create_wp_post', 10, 1);

function mytheme_create_wp_post($form)
{
    $title = $form->getValue('iphorm_1_1');
    $content .= 'Δώρα διαγωνισμού: ' . $form->getValueHtml('iphorm_1_30') . '
';
    $content .= 'Κατηγορία δώρων: ' . $form->getValueHtml('iphorm_1_39') . '
';
    $content .= 'Ημερομηνία λήξης: ' . $form->getValueHtml('iphorm_1_8') . '
';
    $content .= 'Ώρα λήξης: ' . $form->getValueHtml('iphorm_1_9') . '
';
    $content .= 'Διοργανωτής: ' . $form->getValueHtml('iphorm_1_36') . '
';
    $content .= 'Περιορισμός (περιοχή): ' . $form->getValueHtml('iphorm_1_15') . '
';
    $content .= 'Απαιτεί λογαριασμό Facebook: ' . $form->getValueHtml('iphorm_1_26') . '
';
    $content .= 'Όροι διαγωνισμού: getValueHtml('iphorm_1_32') . '">' . $form->getValueHtml('iphorm_1_32') . '
';
    $content .= 'Πληροφορίες διαγωνισμού: ' . $form->getValueHtml('iphorm_1_35') . '
';
    $content .= 'Link διαγωνισμού: getValueHtml('iphorm_1_11') . '">' . $form->getValueHtml('iphorm_1_11') . '
';


    $post = array(
        'post_title' => $title,
        'post_content' => $content
     );


   // Insert the post
    $post_id = wp_insert_post($post);

   // Insert the Custom Fields
    add_post_meta($post_id, 'dwra_diagwnismou', $form->getValue('iphorm_1_30'));
    add_post_meta($post_id, 'kathgoria_diagwnismou', $form->getValue('iphorm_1_39'));
    add_post_meta($post_id, 'diorganwths_diagwnismou', $form->getValue('iphorm_1_36'));
    add_post_meta($post_id, 'hmeromhnia_lhkshs', $form->getValue('iphorm_1_8'));
    add_post_meta($post_id, 'wra_lhkshs', $form->getValue('iphorm_1_9'));
    add_post_meta($post_id, 'periorismos_perioxhs', $form->getValue('iphorm_1_15'));
    add_post_meta($post_id, 'syxnothta_diagwnismou', $form->getValue('iphorm_1_31'));
    add_post_meta($post_id, 'apaitei_logariasmo_facebook', $form->getValue('iphorm_1_26'));
    add_post_meta($post_id, 'link_diagwnismou', $form->getValue('iphorm_1_11'));
    add_post_meta($post_id, 'oroi_diagwnismou', $form->getValue('iphorm_1_32'));
    add_post_meta($post_id, 'plhrofories_diagwnismou', $form->getValue('iphorm_1_35'));