There are generally quite a few rules which limit retrocredit. The most famour one being that you need to be a member of the FFP when you take the actual flights. So be sure to register timely with Oz. All modern databases to further time-stamp their entries and the correct human error with sweeps ... meaning that the flights will count exactly at the dates they were flown, not when you credited them. They might briefly contribute to the current status but only up to the next sweep of the system.