Auto/number - date on particular cell, not whole row

I'm trying to find a way how to populate a datestamp in a cell based on the last change from an another  cell in a particular column



Row 1 - column A: dropdown (Start, pending, stop)

Row 1 - Column B: I want the date when Status changed on column A


Auto/number - date: works, but it looks at every cell on that row, i want it to look within one particular cell.