I've implemented complex logic on enterprise-grade applications serving hundreds of thousands of people and storing tens of millions of rows of data, while making the end result pretty to boot. As long as I receive a clear set of specs, your calculator should be good to go in hours. However, I bid 3 days to ensure that you have an opportunity to throughly test and experience the end product and get back to me on any issues (there shouldn't be any, as I thoroughly test all my own work, but it's best to be overly-cautious).