vacation accrual based on completion of 90 day probation

I know it appears simple, but I just can't seem to wrap my head around it right now. I previously have worked for large corporations and now I'm working for a small company. I'm actually working backwards. They have been in business for 4 years now & still haven't put anything in place for 45-50 employees vacation calculations.
In Column C needs to be the last day of the 90 day probation and the start of the accrual.
In Column D needs to be how many hours accrued to date.
In Column E needs to be how many hours used. This I will populate from our database.
In Column F needs to be the balance of D & E.
I hope this helps. Thank you for your time & help.