The Due Date is calculated based on the Terms code selected for the invoice (which defaults to the terms code selected for the customer).

If you want the invoices to be "due" immediately, you need to create a Terms code with 0 days and set the appropriate customers -- or all of them -- to use it.

I've never heard of a company charging interest from day 1. What line of business are you in that your customers have to pay interest even if they pay the invoice the next day?

You might want to explain how your business charges interest, if it is different from what I wrote above.