One can only backdate a customer invoice date (CIN date) to the customer shipping advice date (CSA date) - or associated document for example: sales order date (COA-date). 

Example: If the customer shipping advice date (CSA date) is the 1st of March, then the date 28th of February cannot be set as the customer invoice date (CIN date).

If the customer invoice (CIN) has several lines linked to different customer shipping advices (CSA's), then it is 

the date of the last customer shipping advice (CSA) shipped from the warehouse that is used.

An error message will be thrown if trying to backdate further than what is allowed: 
'Date cannot be earlier than connected shipping advice)'.

The advantage of this is that both cost and income are booked in the same period.

The user has the option of using advanced invoicing via the Advanced invoice plan and more information regarding this can be found here: Create an advance invoice plan

Link to our webinar regarding how to backdate of sales invoices/credit notes here:Backdate Sales invoices and credit notes