Programming work for loan calculations

We currently run a small loan book using only Excel. We have a separate file for each loan together with a loan book summary and we want to be able to integrate everything into one combined loan book. We need someone to be able to write formulas to calculate interest payment on loans. The interest rate on the loan may change if the loan is not repaid by a certain date or if a repayment is not received on time. Ideally we want to be able to enter one date in the loan book summary and for all loans to be calculated to that date if we need to and for the current position of the company to be calculated at that date - outstanding capital, interest owed, etc. Please only respond if you have have had specific experience in this field and please provide details of such experience. Many thanks