j'ai trouvé la solution. voici le code :
Private Sub ComboBox1_DropButtonClick()
If ComboBox1.ListCount = 0 Then
ComboBox1.AddItem ""
ComboBox1.AddItem "1 "
ComboBox1.AddItem "2 "
ComboBox1.AddItem "3"
ComboBox1.AddItem "4"
End If
End Sub
Private Sub ComboBox1_Change()
TextBox1.Value = Format(Now, "yyyy-mm-dd hh:mm:ss")
End Sub