absence management based on the length of the service

Hi Everyone,

I have a requirement in the absence based on the length of service balance should be provided.

Plan term ---> Jan to Dec 

Balance should be front loaded.

length of service = 0 to 1 year = 20 

length of service = 1 to 2 year = 21 

(i.e)Employee completing the 1 year of service in the mid of the year (June), need to update the balance as 21 mid of the year. 

question : How to achieve this?