Accounting for headwind is everything.

On the way out here United has a game they play where you have to guess the time (hour, minute, and seconds) that the plane would be geographically halfway to Hawaii. They gave you cruising speed, altitude, and headwinds, and you had to give them the time. Get this. I guessed 2:23:37 and the actual time was 2:23:10. I only missed it by 27 seconds which was close enough to get me the first prize. Mmmmmm, chocolate covered macadamia nuts are delicious.