whmcsTools/move_invoice_due_date.php

<?php

// this hook offsets the due date of an invoice using order grace days

if (!defined("WHMCS"))
    die("This file cannot be accessed directly");

use WHMCS\Database\Capsule;
function get_grace() {
    $grace = Capsule::table('tblconfiguration')->where('setting', 'OrderDaysGrace')->first();
    return $grace->value;
}

add_hook('InvoiceCreation', 1, function($vars) {
    $invoice = localAPI('GetInvoice', ['invoiceid' => $vars['invoiceid']]);
    $dueDate = date_create($invoice['duedate']);
    $invoiceDate = date_create($invoice['date']);
    if($dueDate <= $invoiceDate) {
        $grace = get_grace();
        array_shift($invoice);
        date_add($invoiceDate,date_interval_create_from_date_string("$grace days"));
        $invoice['duedate'] = date_format($invoiceDate, 'Y-m-d');
        $result = localAPI('UpdateInvoice', $invoice);
        if($result['result'] == 'success') {
            return true;
        }
        logModuleCall(
            'InvoiceCreation',
            __FUNCTION__,
            $result,
            "Hook Error",
            $invoice
        );
        return false;
    }
    return true;
});

?>