In FINT transaction the interest calculation start date is not available in the selection screen which was available in the F.24 transaction. This setting of date from which the interest will be calculated is handled through the below SPRO customization:
In the preparation for interest calculation we have the options of selecting the Ref Date, as shown in below screenshot. This Reference date is nothing but the date from which the interest can be calculated.
Note: Normally the checkbox "Always Calculate Int. from Net Dte" gives the error " "Do not select "Calc.intrst from net due date" for this int.calc.type Message no. F7128"
However mere selecting the Ref Date as 1 can solve the problem. See the below example.
A document is posted with document date as 01.01.2015
The payment terms are FB30 ( 30 days from the baseline date)
that means the net due date is 31.01.2015.
Transaction FINT executed for this document
Execute and check the Interest Start from Date :-
If you wonder why the date is not 31.01.2015 but is 03.02.2015 it is because in the interest indicator setting you have mentioned transfer days as 2. (Transfer days are the days required for a payment to go from the payer to the payee)
Hence the interest start date is now the net due date of the invoice document.