I need to calculate thousands of values, and the way I need to this is the following:

1) Each value is a debt that was contracted at a specified day, and this may differ from debt to debt
2) The tax is 3% month for every debt
3) I need to know how much these debts were worth at a specified time in the past.

Example: One debt of 104563USD was contracted day 16 november 2008, and I need to know how much it was worth at day 9 july 2009. And the same with many others, but the date the debt took place changes from debt to debt.
I'm doing it manually with HP12c, but it is taking toooo long =/
Any help is appreciated!