All sheets in a file are saved at the same time. I don't believe you can save one sheet and not all the others.
Maybe it would make a difference if my question were worded to detect and plug the date that sheet was changed (instead of saved). Any takers on that one??
I know you said you would rather have a formula than a macro, but this goes well with the "Worksheet_Change" event handler.
Use the code:
ActiveSheet.Range("A1").Formula = "=TODAY()"
Hope that is a start.
How about..
Private Sub Workbook_SheetChange(ByVal Sh As Object, ByVal Target As Excel.Range)
Sh.[A1] = Date
End Sub
This goes in the workbook module. Not a formula (Doesn't update when calculation is done), just updates when a change has been made in a specific sheet.
Juan Pablo