Such an application cannot just pull numbers out of thin air. To be done "right," it would have to interface with your payroll/timekeeping system in order to know know how much leave time has been accumulated and how much has been used, with the accumulated leave time being dependent on employee classification (hire date, part-time or full-time, hourly or salaried, etc....).
I would start by talking to your payroll people and find out how such data is maintained, and how (or even if) you can access it from your intranet. That will likely be the driving force behind how you go about this task. I suspect the likelihood of finding something on the web you can just "drop in" to access this data is not very high.