You might not want to do it in one formula.  Consider doing it in 2 or 3
formulas which will give you more detailed information anyhow.  I'll leave it
to you to set up the general formula to calculate the interest at any given
date.  You have things set up like this, or so it seems

Column A = payment
Column B = date
Column C = remaining balance (after payment in A was made?)
Column D = Total Interest (over entire period or just last period?)

At any given point in time you owe some interest on the outstanding balance.
Calculate that first.  Simple formula (formulas shown as they would be in
row 2)
Most likely this fits into column D
=.01*D1
Of the payment made, a piece of it goes towards the interest, the remaining
goes toward the principal.  You can record the amount that went towards the
balance in Column E using
=A2-D2
for the value in C, I think it looks something like this:
=C1-A2+D2
although it might be =C1-A2+D1, depending on where you're bringing the
interest back into the amount to be used as Remaining Balance w/Interest to
be used as the basis for calculations next month.

You might want to look at some of the financial functions built into Excel
such as PMT(), FV() and PV().  Although you may not feel they are entirely
applicable to your situation.