In Rambase, the rule is that the invoice date is set equal to the registration date.

Excerpts from Sticos (Sticos applies for Norway):

"Invoice shall be issued as soon as possible and no later than one month after delivery, cf. the Accounting Regulations section 5-2-2 and the Tax Directorate's Report AV 8/87 no. 13. See also the Tax Directorate's information of 29.06.2001."

However, a solution has been added where one can change the date / due date and period on an invoice after the customer invoice is registered to status 4.

To change the period, one has an option in the context menu:
  "Change period on sales invoice (finance manager)".

Remember period and invoice date must be coincident.

If the date is changed to a new month, remember to change the period on the invoice accordingly.

(it is the period that determines when invoice is posted in RamBase).

The due date is first set when registering an invoice.

When the DueDate field is blank, it automatically selects the invoice date + current payment terms (for example: 14 days)

If you enter another date in the DueDate field, this will override the payterm.