What this is about
Not an XRechnung rule, but German VAT law (§ 14 (4) no. 6 UStG): an invoice must state the date of delivery or service (BT-72) or the service period (BG-14). Without it the recipient may not deduct input tax — and will ask.
What to do
Enter the delivery date or period. If it equals the invoice date, take the invoice date as delivery date.
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