The bad news is excel only supports dates since 1900.

But John Walkenbach has an addin that will help you:
http://j-walk.com/ss/excel/files/xdate.htm

Bob Martin wrote:

I'm attempting to create an Excel timeline of battles and major eveins in
United States history. I want one of the column's to show how many years it
has been since that date.
For example, say A6 has July 4, 1776. Cell A1 has the formula =Today()
Every time I use the formula =Year(A1)-Year(A6) I get an error message in
the cell.

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Dave Peterson