Alttaki kodu dener misiniz?
Kod:
Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Address = "$A$1" Then
[$B$1] = DateSerial(Year([$A$1]), Month([$A$1]) - 1, 15)
[$c$1] = DateSerial(Year([$A$1]), Month([$A$1]), 14)
End If
End Sub