How to make futur termination date visible before ...

We are storing End Date for Fixed Term Contract employees in a custom date field.

we have a requirement to also store for unfixed term contract employee, the termination date when these one is being entered and make it visible 31 days before the effective date.

2 cases can happen:

case 1 : employee futur termination < = 31

between termination date data entry and termination date the custom date is filled with termination date

case 2 : employee futur termination > 31

between termination date data entry and termination date -32

no date is visible in custom date field

between termination date-31 and termination date

custom date is filled with termination date.

i’m not sure what trigger field i should use in a business rule.

i was thinking to use post save trigger and create a new event post the actual event date ( and prior the futur termination event) but these would requires to also handle the case when termination event is being removed ( these case would also need a post save rule to delete the event with termination date before itself)

am i going in the right direction or is there a simpler way to do so ?

i m thinking also to use offcycle event batch.