Sabbatical eligibility date

Summary

Ways to create a future dated sabbatical eligibility date that is static based on start date

Content

We have created a field for Sabbatical Date that will input the hire start date for their eligibility 5 years out. We need to have the ability to update this every five years with a new date calculation and want to ensure the date is static even when an employee transfers roles. The primary need is for visibility to the employee and developing a simple (or automated) process for updating employee's who hit the 5 year mark. 

Is anyone using this type of field to calculate time-off service dates?