Set Due Date of Task Created From Custom Action

I have a custom action to create a milestone called 'Sprint', and when that milestone is created I also have it creating two tasks: sprint planning & sprint retrospective. I have no issue setting the start date of 'sprint planning' as the start date of the sprint milestone. The part I am having trouble with is setting the start date of the 'sprint retrospective' task to be the same as the last day of the sprint milestone. When I use 'due date' it starts the task the day after the milestone completes.  How can I set a formula to allow the task date to set on the last day of the milestone?