With Sheets("Feuil1")
dat = .Cells(i, 11).Value
For i = 2 To lastrow
If .Cells(i, 12).Value = "3 ans" Then
.Cells(i, 15).Value = DateAdd("yyyy", 3, dat)
ElseIf .Cells(i, 12).Value = "2 ans" Then
.Cells(i, 15).Value = DateAdd("yyyy", 2, dat)
ElseIf .Cells(i, 12).Value = "1 an" Then
.Cells(i, 15).Value = DateAdd("yyyy", 1, dat)
Else: .Cells(i, 15).Value = ""
End If
Next i
End With