Data from new fields in validation form

Hi,

I added a field in my validation form

fields[‘billing’][‘birth_date’]

But I have no idea how to get it to use it and even no idea where is it stored as it doesn’t seem to be in my data base.

function sauvegarder_date_naissance_commande($order_id) {
$date_naissance = isset($_POST[‘billing_date_naissance’]) ? wc_clean($_POST[‘billing_date_naissance’]) : ”;

function sauvegarder_date_naissance_commande($order_id) {
    $date_naissance = isset($_POST['birth_date']) ? wc_clean($_POST['birth_date']) : '';
}
add_action('woocommerce_checkout_update_order_meta', 'sauvegarder_date_naissance_commande');

Do you see where it could be wrong ?

Thank you