Imagine someone runs past you at a substantial fraction of the speed of light. As this happens, you look at your watch to time the amount of time it takes them to run between two points. According to your watch, it took them 5 seconds to make the trip. Later the person tells you that, according to their watch, it took them only 3 seconds to make the trip. Whose watch gave the real time it took to make the trip? How do you know?