I bet you were using an addin by John Walkenbach:
http://j-walk.com/ss/excel/files/xdate.htm



Help with scheduling issue wrote:

I finally entered the world of Windows 7, albeit only because my laptop died.
I am a historan who needs to calculate dates into the 18th century. Xdate
always worked just fine in Excel 2003, but when I load it into the new
computer, Windows 7 sees it as an old process and refuses to recognize it. I
need those date calculations, and I'm not thrilled with buying a later
version of Office - if it even is in the newer versions - because A) I don't
otherwise need it, and B) I've seen the recent version of Office and want no
part of it.

--

Dave Peterson