Calculation pro rata & Termination date

Hello everyone,
We have a customer whose bonus is calculated on a pro rata basis. The compensation plan is structured with an annual cycle from January to December. What happens if an employee changes jobs during the year and their compensation is adjusted? Can the system store a calculation for this, or how do you handle it? 
In addition, a field with the termination date would be helpful if the employee resigns or retires, for example. Has anyone ever configured this or has any experience with it?

 Thank you very much!