Error in formula will saving
 

When a user is saving a workbook in Excell 2000 with many
calculations and references she is getting the following
error:

"A formula in a cell (name:year) could not be converted
because it contains a function that is not available in
the fileformat to which you are saving. Has anybody out
there had any experience with this error. And what is
cell (name:year)?

Regards-

Brady Snow
Mckinney, Texas