Crystal Report Calculation - Urgent

I need to calculate some field based on the date. For Ex;

Balance Forward 8,792,038,358.57


Principal Paid New balance
1-Sep-10 21,224,226.52 8,770,814,132.05
2-Sep-10 12,838,569.65 8,757,975,562.40

Here we have balance forward which is 8,792,038,358.57. We need to substract 21,224,226.52(principal Paid) from the balance forward to generate 8,770,814,132.05 (New Balance) for Sep-1. Now when there is sep-2 then we need to subtract prioncipal paid for 2 Sep from New balance for Sep 1 and keep on going for other dates.