Private Sub Worksheet_Change(ByVal Target As Range)
Dim t, i&, dat As Date
t = Intersect([A:B], Me.UsedRange.EntireRow) 'au moins 2 cellules
For i = 2 To UBound(t)
If IsDate(t(i, 1)) Then
If Int(t(i, 1)) <= Int(dat) Then MsgBox "Date refusée !", 48: Application.Undo: Exit Sub
dat = t(i, 1)
End If
Next
End Sub