It is very normal that people work on their business ideas, talk to potential customers and then refine the idea until they (customers) are ready / willing to pay.
Question is: If a company is going to be registered tomorrow, is it legal to charge the customer (send the invoice) for previous months e.g. previous 6 months as the work was done to refine the idea and develop the features during that period?
Customer has no issue with that, they are ready to pay.
Your queston is a bit complicated as you are in a gray area.
Typically work you have completed before comapny registration is personal b-income (consultant), and should be an income in the period that you have earned it. This is also to avoid issues regarding unemployment benefits etc.
However if you had no guarantee that your work would finalize as an income, one could say that you hours were development or additions to your own skills that have made it possible for you to startup a business providing the exact service. In this case you have no income before you sell a product/service. But if you are on unemployment benefits the case might be more complex.
I have just talked to the SKAT and VIRK. They said it is not possible to charge for previous period where company was not registered. However, on invoice if prevous period is not mentioned but just the item e.g. Work charges without period then it should be fine.
I agree with Argin that it's quite a grey area, but according to the statement you got form Skat, you can't. Nevertheless it's possible to register back in time in order to take your business related expenses into your accountancy. So, this would go for income as well.
Just curious: What's the difference to you?
Virk.dk does not allow to register more than 14 days back in time. According to virk.dk service center, Only foreign companies can be registered more than 14 days back in time.
Our problem is that customer needs invoices where each of the previous month is listed.