You could do a quick IF statement saying "if cell_number equals X, put today's date in", but... That will update every time the spreadsheet is opened. I doubt seriously you want that to happen.

In order to have a date stay set to the date when the tech put the X in there, some VBA will likely need to be involved.

You might consider deleting this post (save the text of it first heh) and re-post indicating (in your title) that you're looking for some VBA code that will enter the current date (and/or time?) when someone fills an X into a cell.

And, also, include the same bit of info in your new post wording too ... that you can't use NOW() or TODAY() because those would always update every time the sheet opens ... instead you need the date (and/or time?) to stay static based on when the tech puts the X in.