Private Sub Worksheet_Change(ByVal Target As Range)
Application.EnableEvents = False 'désactive les évènements
If Not IsDate([A1]) Then [A1] = "=TODAY()"
If IsDate([A2].Value2 & "/" & Month([A1]) & "/" & Year([A1])) Then
[A2] = DateSerial(Year([A1]), Month([A1]), [A2])
Else
If Not Intersect(Target, [A2]) Is Nothing Then [A2] = ""
End If
Application.EnableEvents = True 'réactive les évènements
End Sub