Where can one lose or gain a day?

Time is measured east and west of the prime meridian (0⁰ longitude), an imaginary line that passes through Greenwich, England. East of Greenwich, the time advances by one hour for every 150 of longitude. To the west, one hour is lost for every 150 of longitude. At 180⁰ East, the time is 12 hours ahead of Greenwich, while at 180⁰ West, the time is 12 hours behind Greenwich.

But 180⁰ East and West are the same line. So, there is a difference of 24 hours between the two sides of this line, which is called the International Date Line. Crossing it from west to east, you gain one whole day, but going from east to west, you lose a day. The International Date Line does not follow the 180⁰ line of longitude exactly. It is adjusted so that it does not pass through land areas.