FF to increase the utlization of comp off from 12 to 15 months

We have a group of Staff Accountants who undergo a 2‑year articleship program. For this job, any Compensatory Offs accrued during their first year of service should remain usable for up to 15 months(for now it is working on anniversary year logic that is 12 months) from their date of joining. For the second year of articleship, the validity of their Comp Off balance should expire at the completion of that service year).

Can someone suggest a fast formula to control this logic or even if it is feasible from functional end .