Hi
This can't be done by a formula. You'll VBA code for this. Enter the
following in the worksheet module for your sheet:
Private Sub Worksheet_change(ByVal Target As Range)
If Intersect(Target, Me.Range("A2")) Is Nothing Then Exit Sub
If Target.Cells.Count > 1 Then Exit Sub
On Error GoTo CleanUp
Application.EnableEvents = False
With Target
If .Value <> "" Then
.Offset(-1, 0).Value = Now
Else
.Offset(-1, 0).Value = ""
End If
End With
CleanUp:
Application.EnableEvents = True
End Sub
Format the cell A1 according to your needs
HTH
Frank