How to Accrue Absence for every completed 18 calendar Days?

Summary

How to Accrue Absence for every completed 18 calendar Days?

Content

Hi All,
We have a requirement like below.
every 18 days after hire date employee 1 day should entitlement. On the day he/she completes 18 days 1 day should accrue. If the employee didn't use the accrued balance that balance should add to a new balance. for example

Employee Hire Date: 2019/05/10
As at 2019/05/28 -  1 Day
As at 2019/06/15 - 1 Day ---- Total Entitlement 2 Days
As at 2019/07/03 - 1 Day ---- Total Entitlement 3 Days
As at 2019/07/21 - 1 Day ---- Total Entitlement 4 Days
...............................
Please be kind to share your valuable knowledge. 

Best Regards
Mithun Abhisheka