[FEATURE REQUEST] E-invoice-previews for recurring invoices with variable invoice-dates

Hello!

To get the chance to validate and check future recurring e-invoices it would be nice to have a preview-function, in which one can define the future invoice-date. That should be possible for ZUGfERD-formated e-invoices, too.

Background: The service period in php-date-format should be testable like all other parts of the invoice, too.

I guess it's a lot of work, but I think that can be preventing much afterwards trouble.