The only question I would have asked is whether you'd confirmed with your business side that the interest is compounded monthly and not daily or continuously. To support daily compounding, you would add calculations for the number of days in each month and, of course, divide the annual rate by 365 (and maybe 366 in leap years if that's the way the rate was defined). "Continuous" compounding would involve using a formula based on powers of e, the root for natural logarithms.

It seems clear that your requirements were for monthly compounding, regardless of the number of days elapsed, but I thought it worth while for others reading this thread to be made aware of possible wrinkles.