Originally Posted by Tyger7
Maybe someone can try to do this and see if the achievement can unlock for a passenger/gunner?
I can confirm that the Passenger and Gunner WILL
unlock the achievement. I (the driver) drove the Warthog off the cliff at exactly 12:00 and my two friends were in the Warthog. We all live in the same state so our times were synced up (one friend was 35 seconds ahead of us), but they both got it.
So all in all, as long as it's midnight for all the players, it doesn't matter if you're driver or not.