Time change happens because the Earth spins. This means that different parts of the world are facing the sun at different times. For example, when it is morning in the United States, it’s late at night in Japan.
If you take a non-stop flight from New York City to Johannesburg, South Africa at 9:00 pm, you are on a plane for about 18 hours. When you arrive, your watch will say 3:00 pm. But, because of the time difference, it will be 10:00 pm in Johannesburg.