Each guide shows the no-add-on approaches first — a formula, an Apps Script trigger — then how SmartStamp does the same thing from a panel, with the edge cases handled.
Add a timestamp when a cell changes
Why =NOW() never stays put, and three ways to freeze the
moment of an edit: a self-referencing formula with iterative
calculation, an onEdit Apps Script trigger, and
SmartStamp. Includes the limits of each.