That's the crux of the question breh. Theoretically if you are using GMT if you go back and forth then the end time should be the same as the starting time. What you're missing however is that time is directly proportional to distance traveled, famously expression via the Newtonian derivative distance = velocity x time ( d = v*t). so if we are withholding acceleration from the equation then if you have travel doubled the distance it has taken you double the time. So there's no way it can be 12 in reality, however logically it would follow that if I was in a GMT - 6, I traveled to a GMT -0 zone, then went back directly to a GMT - 6 zone, according to the GMT system the time should be the same as the time I started, because you reversed course. The crux of the issue is that time zones have nothing to do with distance traveled, one is merely a designation, the other is a matter of physics.