I've replaced
$pdf->Cell(0, 6, Lang::trans('invoicesdatedue') . ': ' . $duedate, 0, 1, 'L', '1');
with
$pdf->Cell(0, 6, Lang::trans('invoicesdatedue') . ': ' . date('d.m.Y',strtotime('+15 days',strtotime($datecreated))), 0, 1, 'L', '1');
The "original" invoice is this:
The result is this:
So we can consider this as a "win" for now. I'll search for somebody to write a module, because I'm pretty sure this will get overwritten at the first update and diff-ing is a huge pain in the arse.
Thank you very much for your support!