If I might humbly suggest, you could add to the "Swap Return Calculator" math the option to account for compounding.

So perhaps you might want to specify there that the interest is "not accrued by default", so that people don't get confused. And maybe add a check-box called "compound interest", to calculate the field "Return whole period" based on the compounded interest formula << Future Value = Present Value ( 1+ interest rate )^number of periods >>.