"Persistent", one-time date function

I'd like to see a function like "TODAY()", or cell behavior that supports it, that executes only once when an event is triggered so that the date result that never changes. 

My use case is a Card view Kanban board with status states: Ready, Dev, Test, Deploy, Done, and matching columns for their start dates, e.g. Dev Start.  I'd like to track the dates that the status state is changes; e.g. "=IF(Status@row = "Dev", TODAY())". Current and understandable behavior is that when the status changes to Dev my date cell captures that date, but when the status changes to Test, the Dev Start column goes blank and the Test Start column gets the date.

I want the value that is oroiginally set in the date column to be persistent.

I can see the same feature to capture baseline dates for Start and End dates in schedules..