Keeping persistent dates for status changes

Building a kanban board with typical IT cycle states: Ready, Dev, Test, Deploy, Done.  I'd like to track cycle times and throughput.  I've created (locked) columns to capture a status change for each of the cycle states; e.g. Dev Start and formula "=IF(Status@row = "Dev", TODAY())". That works to capture the date that a task moved to Dev, but when the status changes to Test, the Dev Start column goes blank and the Test Start column gets the date. I understand that would be expected behavior - had to try.

Is there a way to keep the original formula result persistent? That is, once the value is set in the locked column to make it stay, be persistent?