The circle formed by the International Date Line and opposite the prime meridian divides the world vertically into halves. The half west of the prime meridian (and east of the International Date Line) is the western hemisphere. The North American and South American continents lie in the western hemisphere. The half east of the prime meridian (and west of the International Date Line) is the eastern hemisphere. The continents of Europe, Asia, and Africa lie in the eastern hemisphere. Antarctica, which is centered on the South Pole, lies in both the eastern and western hemispheres.

The world is divided into 24 theoretical time zones. The boundaries of some zones are arbitrarily altered to reflect local frontiers and laws pertaining to timekeeping. In addition, certain countries and U.S. states have adopted a modified legal time for part of the year, Daylight Saving Time, during which the "normal" time is advanced by one hour.

Time zones are a measurement of the earth's rotation. Each time the planet completes a full rotation, a single day is completed. An hour is a measurement of 1/24 of a complete rotation. A minute is 1/60 of an hour, and a second is 1/60 of a minute. As the earth rotates, spots on the surface of the earth move away from the sun into darkness, so that night and day are constantly trading places. Time zones compensate for the fact that different parts of the earth are in different positions relative to the sun. Thus, when it is 12 noon in New York, it is 5 p.m. in London.

Local time around the world is determined in relation to the prime meridian which, as noted above, passes through Greenwich, England. The time at the prime meridian is called Greenwich Mean Time (GMT). In other time zones, the local time is calculated by adding one hour for every time zone east of the prime meridian, or subtracting one hour for every time zone west of the prime meridian.

The local time directly over the International Date Line is +12 hours relative to GMT. The local time in the People's Republic of China is +8 hours. East of the Date Line, California is -8 hours, or 8 time zones west of the prime meridian, and New York is -5 hours.

The International Date Line opposite the Prime Meridian is +12 hours. Thus, when it is 12:00 p.m. on January 1 in Greenwich, England, it is 12:00 a.m. on January 2 at the International Date Line. A traveler crossing the date line from east to west will seem to "lose" one day, and a traveler crossing the date line from west to east will seem to "gain" one day. For example, if an airline flight departs from Honolulu on 15 September, it will arrive at Sydney on 16 September, assuming the flight arrives before 12:00 midnight on the day it departs. If the flight arrives after midnight, it will arrive on 17 September, creating the illusion of "losing" two days. In contrast, if a flight departs from Sydney on 22 September and arrives in Honolulu before midnight, it will arrive on 21 September. If the flight arrives after midnight, it will arrive on the same date as the departure date.

Local time in relation to GMT (hours)

Addis Ababa +2 Algiers +0 Amsterdam +1 Athens +2 Atlanta -5 Baghdad +2 Barcelona +1 Beijing +7 Berlin +1 Buenos Aires +4 Chicago -6 Copenhagen    +1 Denver -7 Dublin +0 Frankfurt +1

Geneva +1 Helsinki +2 Hong Kong +7 Honolulu -11 Istanbul +1 Johannesburg   +1 Kingston -6 Kuala Lumpur +7 La Paz -5 Lima -6 London +0 Los Angeles -8 Manila +7 Montreal -5 Moscow +3

Mumbai +5 Nairobi +2 New Delhi +4.5 Paris +1 Perth +7 Prague +1 Riyadh +2 Rome +1 Sao Paulo -4 Seattle -8 Singapore    +7 Stockholm +1 Taipei +7 Tokyo +8 Vienna +1