The Earth rotates on its axis once every 24 hours, causing different parts of our planet to face the sun at different times. When it's noon in one location with the sun directly overhead, it's midnight on the opposite side of the Earth. This natural phenomenon is why we need different time zones around the world.
To standardize timekeeping globally, the world is divided into 24 time zones, each spanning approximately 15 degrees of longitude. This corresponds to one hour of Earth's rotation. The system is based on Coordinated Universal Time, or UTC, with zones extending from plus 12 hours to minus 12 hours from this reference point.
When flying from China to the United States, you travel eastward across the Pacific Ocean. China operates on UTC plus 8, while the US West Coast is on UTC minus 8, creating a 16-hour time difference. This eastward journey across multiple time zones is key to understanding why you can arrive on the same day you departed.
The International Date Line runs roughly along the 180-degree meridian in the Pacific Ocean. This imaginary line determines where each calendar day begins and ends. When you cross the Date Line traveling eastward from west to east, you subtract one full day from your calendar. This is the key mechanism that allows flights from China to the USA to arrive on the same day they departed.
Here's a concrete example: You depart Beijing on Monday at 2 PM local time. During the 12-hour flight eastward, you cross the International Date Line, which subtracts one full day. You then arrive in Los Angeles on Sunday at 10 AM local time. Even though 12 hours have passed during the flight, the combination of crossing time zones eastward and the Date Line effect results in arriving on the same calendar day you departed, or even earlier!