I was entering some data in a spreadsheet and wanted to repeat the contents of the cell above. I thought I typed CMD-" but the current time appeared. With a little bit of playing around, I figured out that I had typed CMD-; instead.
OK, how do I get the current date? I went to Excel Help and after searching for several combinations of keyboard, enter, and date I found that one enters CONTROL-;
Now, how often I will use my new-found knowledge is another question.
Warning: I use Excel in Office 2004 for the Mac. Other versions may have different combinations. Software developers are not known for consistency across versions.