Numbers spreadsheet - Today's date update

Hi - I wonder if the forum could answer this simple question. I have successfully used some kind of action in the past to automatically update the Date in a cell within a Numbers spreadsheet to Today's date. In other words, every day, this cell shows the correct date without any intervention from me. But it was so long ago that I do not remember how I did it. Searching through the Help menu and other menus, it is not clear how this is done. Reading comments on Google, it is still not clear how to do it. Right clicking on the existing cell does not reveal how I did it. One would have thought that this is fairly basic functionality in any spreadsheet. Can you tell me how to do it please ? Thanks in advance.