E9.2 Setting up probation period for DBA

I looked in the manuals and through these forums to find this verbiage, but was not successful.

When a new employee is hired, they get a 3 month probationary period.
After these three months are completed, we are manually adding in the previous 3 months of vacation accruals.
We are attempting to move away from manually adding this.

At the moment, we activate the accruals participations after their 3rd month, but this takes extra effort on calculating and entering (also raises risk of wrong data coming in) it also messes with the anniversary date of the employee, and HR needs to recalculate and re-enter data in to fix their anniversary date when they should laps over to the new vacation tier.

We would like for the system to show 0-3 months of no accruals, but once the 4th month hits, it should give them their previous 3 months.