Timestamp when a cell is modified by formula (Excel)

I need a way to mark an adjacent cell whose value changes by a formula. Using this as an example, I need a cell adjacent to A1 in sheet 1 to mark the date and time the cell value changed.

The example I linked to above triggers a message box when a cell value changes by a formula (workheet_change events are not displayed to recognize cell value changes when it contains a formula whose value changes due to a cell change elsewhere). I don't want a message box, but I need a timestamp.

For simplicity's sake, I'll post instructions on this related question here, any additional help with this specific question would be appreciated.

In Sheet1 Cell A1 put this formula

=Sheet2!A1+1

      

        
        
        
      

    

Now in the module paste this code

Public PrevVal As Variant

      

        
        
        
      

    

Paste this into the code area of ​​the sheet

Private Sub Worksheet_Calculate()
    If Range("A1").Value <> PrevVal Then
        MsgBox "Value Changed"
        PrevVal = Range("A1").Value
    End If
End Sub

      

        
        
        
      

    

Finally, in the Code for this book field, paste this code

Private Sub Workbook_Open()
    PrevVal = Sheet1.Range("A1").Value
End Sub