As part of setting up a worksheet, you may want the header or footer to contain the date that the workbook was last edited. Excel doesn't maintain this information, but it does allow you to perform macros whenever certain events occur, such as changes to a workbook. All you need to do is add a macro such as the following to the ThisWorkbook object in the VBA Editor:

Private Sub Workbook_SheetChange(ByVal _
  Sh As Object, ByVal Target As Excel.Range)
    ActiveSheet.PageSetup.CenterFooter = _
      "Worksheet Last Changed: " & _
      Format(Now, "mmmm d, yyyy hh:mm")
End Sub

The macro results in each footer on each worksheet in the workbook having separate dates and times on them, since each worksheet would be updated at different times. You can change the destination property (.CenterFooter) to one of the other header or footer properties (.LeftHeader, .CenterHeader, .RightHeader, .LeftFooter, .RightFooter) as desired.

You may want the header or footer to instead include the date that the workbook was last saved. (This is what many people really view as the "last edit date.") The information is visible in the Properties dialog box for a worksheet, but Excel has no menu selection or other command that allows you to insert this date into a header or footer. Instead, you must use a macro to add the desired information.

The best way to accomplish the task is to add a macro to the ThisWorkbook object that is triggered just before a workbook is saved:

Private Sub Workbook_BeforeSave(ByVal _
  SaveAsUI As Boolean, Cancel As Boolean)
    Dim sht
    For Each sht In Sheets
        sht.PageSetup.CenterFooter = _
          "Workbook Last Saved: " & _
          Format(Now, "mmmm d, yyyy hh:mm")
    Next
End Sub

This macro steps through each worksheet in the workbook and changes every center footer to have the date that the workbook was saved.