Adding a field for delivery-date and performance-period

As it's a legal mandatory I miss a field for the delivery-date and as an alternative a service-period (Lieferzeit aka Lieferdatum or as an option Lieferzeitraum). This information is mandatory for e-invoices, at least for correct invoices after German VAT-law. I used a custom-field for this information, but I cannot set up this field to the needed e-invoice-tag/xml-value.

I hope you understand my broken English.

However: It would be a good addition to add a field for a delivery-date with the option of service-periods like date to date.