Hi Sylvia,
I would like give you simple repro steps on the calculation of Interest:
1. Set the parameter for interest calculation as follows:

2. Create an interest code with the following setting:

3. With the ranges on the day:

4. Set that interest code on the customer posting profile of the customer used for test

5. Now create a free text invoice for a customer connected to the customer group, in my example EXT.
On the invoice set invoice date and due dateto 1st of January 2019.
Invoice amount including VAT 10.000

6. Now since the customer hasn’t paid the invoice we would like to send an interest note to the customer. The first one is sent the 31st of January, and the customer still hasn’t paid the invoice.
Go to credit and collections and create interest notes I have only selected the customer to which the free text invoice was sent.

6. Customer transactions:

I hope this helps.
Regards,
Sheela
Support Engineer- Dynamics- 365 F & O