Macro + 6 mois

Function extraire_date(X)
Dim i As Long, D As Date

extraire_date = Null
For i = 1 To Len(X) - 7
  If Mid(X, i, 8) Like "##/##/##" Then
    On Error Resume Next
    D = CDate(Mid(X, i, 6) & "20" & Right(Mid(X, i, 8), 2))
    If Not IsError(D) And D > 0 Then
      D = DateSerial(Year(D), Month(D) + 6, Day(D))
      extraire_date = D
      Exit Function
    End If
  End If
Next i

End Function

=SI(ESTNUM(date);DATE(ANNEE(date);MOIS(date)+6;JOUR(date));"")

Sub DatePlus()
Dim c As Range, t As String, i As Integer, dat As String
Application.ScreenUpdating = False
For Each c In ActiveSheet.UsedRange
  If Not IsDate(c) And c.Text Like "*##/##/##*" Then
    t = c.Text
    For i = 1 To Len(t) - 7
      dat = Mid(t, i, 8)
      If dat Like "##/##/##" And IsDate(dat) Then
        With c.Offset(, 6)
          .Value = DateSerial(Year(dat), Month(dat) + 6, Day(dat))
          .NumberFormat = "dd/mm/yy"
          .HorizontalAlignment = xlCenter
          .Interior.ColorIndex = 6 'coloration facultative
        End With
        Exit For
      End If
    Next
  End If
Next
End Sub

Private Sub Worksheet_BeforeDoubleClick(ByVal Target As Range, Cancel As Boolean)
If Not Target.Text Like "*##/##/##*" Then Exit Sub
Dim t$, i&, dat$
Cancel = True
t = Target.Text
For i = 1 To Len(t) - 7
  dat = Mid(t, i, 8)
  If dat Like "##/##/##" And IsDate(dat) Then
    Target.Interior.ColorIndex = xlNone
    Target.Font.Color = 11297280
    With Target(1, 7)
      .Offset(, -1) = Target(1, 0)
      .Value = DateSerial(Year(dat), Month(dat) + 6, Day(dat))
      Cells(Target.Row, "B") = .Value
      .NumberFormat = "dd/mm/yy"
      .HorizontalAlignment = xlCenter
      .Interior.Color = 65535
      .Font.Color = 255
    End With
    Exit For
  End If
Next
End Sub

Private Sub Worksheet_BeforeDoubleClick(ByVal Target As Range, Cancel As Boolean)
If Not Target.Text Like "*##/##/##*" Then Exit Sub
Dim t$, i&, dat$
Cancel = True
t = Target.Text
For i = 1 To Len(t) - 7
  dat = Mid(t, i, 8)
  If dat Like "##/##/##" And IsDate(dat) Then
    Target.Interior.ColorIndex = xlNone
    Target.Font.Color = 11297280
    With Target.Offset(, Val(Target(1, 0)))
      .Offset(, -1) = Target(1, 0)
      .Value = DateSerial(Year(dat), Month(dat) + Val(Target(1, 0)), Day(dat))
      Cells(Target.Row, "B") = .Value
      .NumberFormat = "dd/mm/yy"
      .HorizontalAlignment = xlCenter
      .Interior.Color = 65535
      .Font.Color = 255
    End With
    Exit For
  End If
Next
End Sub

Private Sub Worksheet_BeforeRightClick(ByVal Target As Range, Cancel As Boolean)
Set Target = Target(1)
If Not Target.Text Like "*##/##/##*" Then Exit Sub
Dim t$, i&, dat$
Cancel = True
t = Target.Text
For i = 1 To Len(t) - 7
  dat = Mid(t, i, 8)
  If dat Like "##/##/##" And IsDate(dat) Then
    Target.Interior.ColorIndex = xlNone
    Target.Font.Color = 11297280
    With Target.Offset(, Val(Target(1, 0)))
      .Offset(, -1) = Target(1, 0)
      .Value = DateSerial(Year(dat), Month(dat) + Val(Target(1, 0)), Day(dat))
      Cells(Target.Row, "B") = .Value
      .NumberFormat = "dd/mm/yy"
      .HorizontalAlignment = xlCenter
      .Interior.Color = 65535
      .Font.Color = 255
    End With
    Exit For
  End If
Next
End Sub