Set InvoiceDate based on OrderDate via functions.php

functions.php

<?php
/**
* @snippet WooCommerce - Fortnox plugin by Redlight Media - Set InvoiceDate based on OrderDate via functions.php
* @author Redlight Media AB / Christopher Hedqvist
* @compatible WooCommerce 3.2.5
*/
function redlight_fortnox_payment_method_invoicedate($invoice, $order_id) {
    $order = new WC_Order($order_id);
    
    $invoice['Invoice']['InvoiceDate'] = $order->get_date_created()->format("Y-m-d");
    
    
    return $invoice;
}
add_filter('obj_fortnox_update_invoice_params', 'redlight_fortnox_payment_method_invoicedate', 10, 2 );