I have a value that depreciates monthly by a fixed percentage. My understanding (and this is all based on remembering my highschool maths lessons) is that to calculate the future value I would use the formula:

fv = pv * (1 - (r/12))^p

where:

fv = future value
pv = present value
r = annual rate
p = monthly period

I have done some sample calculations in excel and this appears to work.

Now the problem is I am maintaining a program that calculates it as follows:

fv = pv * (1 + (r/12))^-p

Again doing some calcs in excel shows that this gives slightly different values.

Can anyone hazard a guess as to what the program is actually doing? Is it some form of fancy depreciation?

All help/opinions appreciated.