Aircraft operators need some flexibility when executing flight operations such as taking off, landing or entering into a sector, because the exact time of these operations is often affected by uncertainty. We express this flexibility in terms of time windows which are periods of time assigned to flights under the agreement that when a flight performs all its operations within the corresponding time windows no downstream delay is caused to any other actor of the air traffic system. If some last minute inconveniences make a flight unable to meet its time windows, some delay may also be caused to other flights. Relying on simulated and real air traffic data of the European airspace, this paper shows that this additional delay can be significantly reduced if there is the opportunity to reallocate unused airport or sector capacity, especially when airlines collaborate by updating and disclosing their expected times of departure, arrival and entry into sectors.