try this 

Private Sub Worksheet_Change(ByVal Target As Range)
Application.EnableEvents = False
On Error Resume Next
If Not Intersect(Target, Columns(1)) Is Nothing Then
Target = Year(Now) & "-" & Month(Now) & "-" & Target
End If
Application.EnableEvents = True
End Sub
it works in column A
probably you would have to change "-" into "/" but check this first