Sub modifierdates()
Dim i&, fin&, aa As Variant
fin = Feuil2.Range("C" & Rows.Count).End(xlUp).Row
aa = Feuil2.Range("C2:G" & fin)
For i = 1 To UBound(aa)
If Mid(aa(i, 1), 1, 2) = 21 Then
aa(i, 5) = CDate("01/01/" & Split(aa(i, 5), "/")(2))
Else
aa(i, 5) = CDate("01/" & Split(aa(i, 5), "/")(1) & "/" & Split(aa(i, 5), "/")(2))
End If
Next i
Feuil2.Cells(2, 3).Resize(UBound(aa), UBound(aa, 2)) = aa
End Sub