Calculating Dates.

Hello,

I'm trying to create a spreadsheet using I can calculate dates using different columns. Attached you will a sample spreadsheet. Using column B, I would like calculate the next appointment which will be 90 days from the initial orientation in Column C. However I want the date to change once I put a date in Column D. For example, Name Peachy Strawberry Initial Orientation was conducted on 6/7/15 and her Quarterly is scheduled for 10/7/15. However, the quarterly visit was completed 10/1/15. Is there a formula that could change the Quarterly Scheduled to reflect her next quarterly visit?