Windows may get all the attention, but when you want to get real work done, you turn to the applications that run on it. And if you use spreadsheets, that generally means Excel. Excel is, of course, ...
Press Ctrl+; to insert the current date. Press Ctrl+Shift+; to insert the current time. Press Ctrl+; > Space > Ctrl+Shift+; to insert the current date and time together in the same cell.