bon j'ai fini par trouve mon erreur
cela donne ca
Sub week()
aa = Cells(1, 1)
tmpbis = Mid(aa, 1, 2) & "/" & Mid(aa, 4, 2) & "/" & Mid(aa, 7, 10)
Cells(4, 1).Value = CDate(tmpbis)
Cells(4, 3).Select
ActiveCell.Formula = "=WEEKNUM(RC[-2])&"" - ""&year(RC[-2])"
wek = Mid(Cells(4, 3), 1, 4) + Mid(Cells(4, 3), Len(Cells(4, 3)) - 1, 2)
Cells(1, 3) = "'" + (wek)
End Sub