Goto the VB editor, add a module, then add this code.
Function LastModifiedDate() As Date
LastModifiedDate = CDate(FileDateTime(ActiveWorkbook.FullName))
End Function
Then, select the cell on the worksheet in which you want the date to appear. Format the cell to the date format of your choice. Select Insert > Function. Pick the LastModifiedDate function and hit OK. OK the next screen as well. This will be updated everytime the sheet is recalculated.
Hope this helps! [sig][/sig]