How can I populate milestone dates for more than one outcome excel

I am building a spreadsheet so that when you place a date at the beginning it will then populate the dates for all milestones. At the end of the spreadsheet are three construction outcomes. Now this is easy when it comes to placing the start date into the spreadsheet because each milestone is based off another milestone. I am having a hard time creating this same spreadsheet for the dates going backwards. If we have an end date then we want to know when each milestone should take place with each construction outcome.

Does this make sense?

Attached is the spreadsheet I am working on. The top shows milestones and dates going forward. The green at the beginning of the spreadsheet (top row titled S&L) shows you place a date at the beginning in order to populate dates. The green at the end of the spreadsheet (bottom row titled S&L) shows you place a date at the end in order to populate the dates.