Last save date in excel worksheet footer

I am using Excel 2007 with a workbook that has many sheets. I need to have the date when the worksheet was last saved - in the footer. I was able to find the following:

Private Sub Workbook_BeforeSave(ByVal SaveAsUI As Boolean, Cancel As Boolean)

Dim ws As Worksheet

For Each ws In ThisWorkbook.Worksheets
With ws.PageSetup
.LeftFooter = "Last Save Time: &T"
.RightFooter = "Last Save Date: &D"
End With
Next ws

Set ws = Nothing

End Sub

This changes every worksheet. I need it to only change a sheet that has been edited (so each worksheet can have a different date). Is this even possible? Should I use a cell instead of a footer? Or do I have to create multiple workbooks?

Thanks!