I would like to use data validation to restrict cell input to either a
valid date after 1/1/08, or "N/A". This is the formula I'm starting
with:

=IF((D17="N/A"),TRUE,IF(D17DATE(2008,1,1),TRUE,FALSE))

The problem is, if the user types anything in text (such as "Something
Else"), then Excel calculates the value of the text string is greater
than date(2008,1,1).

What do I need to do to allow only dates or the string "N/A"?

Thanks,
Rachel Garrett