Update rule : Single date issue

Dear Experts,

could you please assist in how to use the single date function?

i tried to maintain the condition manual but for the first flow 31.01.2016 every single time i maintain data in it it erase it and starts from 29.02.2015

as below, even maintaining a new line for the first period but still, the system uses different value which got calculated based on the interest rate.

Regards,

Ahmed Samir