International Date Line
If you see a world map, you will find a line marked as International Date Line connecting the two poles. It passes east of Japan and Australia. It is an imaginary line. Countries to the east of this line are always a day ahead of those to the west. Travellers who cross the line either gain or lose a day, depending on which direction they are going.
The line is so drawn that it generally passes through sea. But it is adjusted through a zigzag pattern if any island occurs on the straight line, thereby putting them either on the east or west.