Develop Simple Website Calculator

I need a simple calculator created that I can place on my career related website (wordpress site). This will be a 'time off calculator" that will determine how much vacation time someone will have at a future point in time based on two inputs. The inputs are current balance of time off accumulated and time off accrual rate (# of hours per week or month). The user should then be able to choose a future date and see how much vacation time they will have. I'd like the calculator to look and work very similar to this one, also on my website at [login to view URL] It just needs a field for current amount of time off (in hours), a field for time off accrual rate (hours per week or month), and a date field the user can choose in the future. Then just display the total amount of time off at that future date in the final field.