Hi Dernan, you can not use either formulas or dates column headers to tables. You are doing nothing wrong, its just how Excel works.

When I am faced with this issue myself what I tend to do is add a new row just above the table headers. As this row is not part of the table, you can include formulas or dates, and you can also use it to drive other formulas that might be included within the table.

I don't think this is the news you wanted to hear, but there are always workarounds in Excel :-)