Gratuity Calculation

Dear All,

Please help me in creating a template to calculate Gratuity for our staff in a simple way with below given details:

Gratuity Rules:
a) Service for first 3 years will be 15 days basic salary per year
b) Thereafter from 4th year 30 days basic salary per year

I want to have fields like Date of Joining, Date of Ending, Basic Salary & Total Gratuity Days & Total Gratuity Amount to be paid. URGENT HELP PLEASE.