date of embarkation

, in relation to a member, means the date upon which the member embarked on a ship or aircraft for service in an overseas area, but shall not be deemed to refer to the embarkation on that ship or aircraft on that date of a member who disembarked from that ship or aircraft before reaching an overseas area and, in the opinion of a prescribed authority, did not proceed forthwith in the same or another ship or aircraft for service in an overseas area.