I appreciate what you've done so far, @pno2nr. Let me share some information on how to get the date field in your invoice.

To begin with, please ensure to turn on the service date feature in the accounts and settings. 

Here's how: 

1. Click the Gear icon, then select Accounts and Settings. 
2. Navigate to Sales and then locate Sales form content. 
3. Click the Pencil, then toggle the Service Date. 
4. Once turned to green, click Done. 

After this, go to Custom Form Styles, then ensure that the date is included in the template. 

Here's how:

1. Go to Custom Form Styles, click New Style, and choose Invoice. 
2. You can select any template, then go to Content.
3. Click the body, then toggle the Date. 

Once done, you can now create an invoice with a service date or date field. 

Additionally, you can check out this article to learn the new estimate and invoice layout: See what’s new with estimates and invoices in QuickBooks Online.

Furthermore, this article can help you learn how to personalize your invoices: Customize invoices, estimates, and sales receipts in QuickBooks Online.

Moreover, once you've received your invoice payment, this article can help you learn how to record it: Record invoice payments in QuickBooks Online. 

Let me know if you need further assistance getting the date field to show in your invoices or QuickBooks. Keep safe.