Yes I need a Function,
To be completely clear, here is example.
The interest rate is changing twice in year.
01. July 2013 - 11 %.
01. Jan 2014 - 11,5 %
01.July 2014 - 10,5%
the calculated interest is sum from interest calculated in all rate periods.
Due date - 01 Sep 2013; Payment date 20 Sep...