I see, that's a good point; the better question would be whether it lets you book anything that isn't the day of departure. The airberlin website (for Etihad coach) says;

"For your return flight from Abu Dhabi International Airport, please use the free airberlin City Check-in at the Etihad Travel Mall, which is available from 24 hours before departure, where you will also receive your boarding pass. Here, you can even book your next flight or holiday, apply for a visa, use the car rental service, book sightseeing tours or buy tickets for Abu Dhabi's fantastic attractions."

I think you might be in luck, but obviously can't say for certain, unfortunately.