Adding Days to a Date

Hello,

I have been trying to figure it out and even tried reading all the questions similar to this one so if anyone can help me better understand it would be greatly appreciated.

I have a field name "StartDate" and a field name "EndDate" I am trying to add Inspection dates.

StartDate + 30 days = 1st Inspection

StartDate + 60 days = 2nd Inspection (If EndDate has passed leave blank)

StartDate + 90 days = 3rd Inspection (If EndDate has passed leave blank)

StartDate + 120 days = 4th Inspection (If EndDate has passed leave blank)

StartDate + 150 days = 5th Inspection (If EndDate has passed leave blank)

StartDate + 180 days = 6th Inspection (If EndDate has passed leave blank)

EndDate + 14 days = Final Inspection

To anyone that can help thank you for your time.

AJ