showing the expiry date

Hi,

I have a file that has start date column. I'd like to show the account's expiration date on another cell but it's going to be dependent on another column. Say
Column A - customers name
Column B - start date
Column C - rate
Column D - End date
Column E - Follow up Date

Rate column will have the ff inputs :
rate 1 = .09
rate 2 = .11

If rate is .09, then end date will be 2 years from the start date
If rate is .11, then end date will be 3 years from the start date

There are several things I need to happen :
1. Show the customer's individual end dates in Column D
2. I'd like to show in column E the follow up date which is 90 days before the end date

I'd appreciate any help from you.

Thank you.