Let's say you have a fast-moving object in space and two points on opposite ends of the earth, one which is currently carried by earth's rotation towards it, and one away. If the scale of time dilation depends on relative speed, then the two points on Earth should have different relative speeds and thus time, however you'd expect their time to be the same. In a short time frame that system would be more or less inertial, no? This is something I never understood