Re: Selective access list to generate a date when changed
I think your question is unclear. You're saying you have selective access enabled so normally only the users allowed to would be able to change the date field. Hence, my misunderstanding. You should just allow them to enter the date. You can also use Dynamic Cell Access (DCA) to lock the status line item when it is approved and only unlock the date line item when the line is approved (and also lock it once the date is entered)
Using the formula definitely cannot work, as formulas are permanently calculated.