Your wide spaceship has inside a light and a receiver, widely separated, sideways to direction of travel. As pilot, you notice the blinking light always takes the same time to hit the receiver, whether you are approaching speed of light or not. But an outside observer notices that the path the light travelled was much, much longer when you are in motion, and therefore perceives it as slower (at near C it creeps along the sidewalls) - so the conclusion is: time is slowing for you as the pilot. I agree that light travels a much longer distance, but to insist that because of the constancy of C, time must be slowing down, is a jump. How well is the constancy of C established ? This must not be confused with the simultaneity experiment: If you were seeing a synchronized huge digital clock on Mars, it would be lagging. Here there is no time compression, the report of time is merely delayed. The absence of an ability to verify simultaneity over huge distances to me does not threaten it as a concept ? We are just finding out late about what time it is, but can still have full confidence in the clock..