Sub test()
Dim MonMois As Byte, MaCel As Range, NumSeq As Integer
Set MaCel = Range("A65536").End(xlUp)
MonMois = Mid(MaCel, 5, 2)
NumSeq = Mid(MaCel, 8, 3)
If MonMois = Month(Date) Then
Range("A65536").End(xlUp).Offset(1, 0).Value = _
"REC/" & Format(MonMois, "00") & "/" & Format(NumSeq + 1, "000")
Else
Range("A65536").End(xlUp).Offset(1, 0).Value = _
"REC/" & Format(Month(Date), "00") & "/" & Format(NumSeq + 1, "000")
End If
End Sub