Several lines of longitude to keep in mind are as follows:
1. 0 degrees longitude, through the town of Greenwich (London, England). Under international agreement, the meridian line is expressed as 0 (zero). Often called Greenwich Mean Times (GMT). Through the 0 degree longitude line, we can calculate the local time at the Earth’s surface with respect to the location of longitude.
2. Longitude 180 degrees, located in the Pacific Ocean called the international date line. When we walked westward and crossed the line should be 180 degrees plus 1 day, whereas if traveling eastbound and crossed the line by that date, the time should be reduced by one day.
For example, when we walk in a westerly direction from San Francisco to Tokyo to be added one day, for example from Tuesday to Thursday in Tokyo. Conversely, when we walk to the east from Tokyo to San Francisco minus 1 day, for example from Tuesday to San Francisco was on Monday, and tomorrow is Tuesday again.