Fix for invoices with date <= 1900

Because the helper script for invoice, (order, packing lists, etc) printout is using DateTime.strftime it does not support dates before 1900. This is a documented behavior in python that dates before 1900 cannot be used.

In these cases it's not supposed to happen, but if by mistake user enter a date before 1900, printing the invoice would fail, which is not good.

These patterns were very simple and did not need strftime.