its hard to get your head around it and it's all to do with relativity (made famous by Einie himself) - if you were to travel in the plane with the clock it will tick once per second exactly, but from the perspective of standing with the clock on the ground, the one in the plane is ticking slower so when you put them together again they show different times: as mentioned before, the faster you go the slower time becomes (but only from others' perspectives) so you appear to slow down in time. Consider as an example you stand looking at a clock - it is ticking and the light is transmitted to you showing you the clock face ticking once per second. Now run away from the clock backwards at the speed of light - ok not realistic but hypothetically if you could do this, the rate you were running away from the clock is the same as the waves of light coming towards you from the clock, and so the clock stops - you have slowed down time (as you sped up towards the speed of light, the clock slowed down)