Private Sub Worksheet_Change(ByVal Target As Range)
Dim DatDéb As Date, An%
If Target.Address = "$B$4" Then
An = Year(Date)
DatDéb = CDate("1 " & Target.Value & " " & An)
[B5].Value = DatDéb
[B6].Value = DateSerial(An, Month(DatDéb) + 1, 0)
End If
End Sub