Hi Rosa,
In the example you sent you are Capitalizing Interest.
In the case of 6-May you are capitalizaing interest from 30-Apr to 31 May but you are on 6th May, so you are adding capital for a time which has not yet passed 7th May to 31th May.
The Ref.Int.Date Rate could be only one in the calculations, so if your system considers 30th April as your interest rate, that will be valid for the whole period (30th Apr-31th May).
You can change the amount of Capital to calculate interest,
for example: Capital is 1.000 and on May 12th you decrease 200, interest since 30th Apr to 11th May will be calculated on a capital of 1000, and from 12th may to 31th May the capital will be 800.
Hope this helps
Kind Regards