Automation based on date which is calculated by formula

Hello, I’m trying to create a scenario where a date is calculated 5 years from an initial date. I then need a date that is 18months prior to the 5yr date, one that is 12months prior and one that is 6months. I then want to trigger an email at those 18/12/6m dates. I can use the formula column to calculate the 5yr, 18/12/6m dates but automations wont pick up the formula columns. It also has to work where the initial date is in the past as we have retrospective data we are bringing into the board.