having trouble in sheets date and time stamping an existing document in numbers

Unless the menu has changed in the current edition of Numbers, or unless I created a keyboard shortcut for this menu item, you'll find the keystroke combination command-; ( Press and hold command, tap the semi colon) will insert a time and date value (with the time part hidden) into a text box, and a double click on the entry will bring up a menu from which you can choose to show the date, the time, or both:

Unfortunately (perhaps), this feature does not work in a table cell.

Regards,

Barry

Select a cell (double click like you are going to type into it), go to the Insert menu and select Date & Time, it will insert the current date and time into the cell. If you need to do this often, you can assign a key combination to that menu item to make inserting it quicker. You can probably find good instructions online or we can walk you through it here or provide a link to instructions.

If you were looking for a function to do the date/time stamp, there is no function that will do it. The functions TODAY and NOW will update whenever a change is made to the spreadsheet and every time it is opened.