Time Zones


Suppose you board a plane and fly eastward from Atlanta, Georgia, leaving at 9:00 p.m. EST. Seven hours later you land in Frankfurt, Germany, in midmorning sunshine with bustling traffic, although your watch says it is only 4:00 a.m..

What happened? Why isn’t it still dark? The answer involves the rotation of the earth. Time changes with distance.

Once people began traveling great distances, they had to grapple with the fact that distance and time are related. People divided the day into 24 hours as long ago as the 14th century. But even in the late 19th century, people continued to operate according to local sun time. As trains began to crisscross the continent, a system for coordinating time became essential.

An international conference in 1884 in Washington, D.C., solved the problem. Time around the globe was standardized against the time at the prime meridian at Britain’s Royal Observatory at Greenwich, England. The world was divided into 24 time zones, each approximately 15° of longitude in width and each differing by one hour from the next. In 1918 the U.S. Congress applied this system to the United States, establishing official time zones.

To understand the time system, you need two tools: a 24-hour clock and a map of the world’s time zones. The 24-hour clock eliminates the a.m./p.m. distinction and provides a different numeral for each hour of the day. The time at Greenwich is called Greenwich Mean Time (GMT also called Coordinated Universal Time).

The time elsewhere in the world is expressed as “plus or minus GMT. Zones east of Greenwich are plus GMT; zones west of Greenwich are minus GMT.

Where do the pluses begin and the minuses stop? The location 180° east of Greenwich that is twelve time zones ahead of GMT is also the location 180 west of Greenwich, twelve time zones behind GMT. So when it is noon at Greenwich on October 14, at this location it would be midnight on both October 14 and October 13! To fix the problem, the point 180° from Greenwich was named the international date line, and the time zone there was divided in half. Thus the world is actually divided into twenty-three full zones and two half zones; the twelfth zone west and the twelfth zone east are each half a zone wide. At any time, two calendar days are in effect in the world. Travelers crossing the international date line in a westward direction, toward Japan, add a day (Sunday becomes Monday). When they return, they subtract a day (Sunday becomes Saturday).

The time at any particular place is called its local time. Using the system of time zones, you can determine the local time for any place in the world. You can also determine how long it will take to get to a destination, or the elapsed travel time.

The calculations can get complicated for some destinations and at certain times of the year. Some countries, such as India, set their standard times 15 or 30 minutes from the time designated by the international system. Other countries, such as China, have a single time zone. Furthermore, many places move their clocks during part of the year to create what is called daylight saving time or summertime. Not all areas, even within the United States, adopt the time change. Of those that do, different countries may make the change on different dates.

To Calculate the Time Difference

  1. For each location, find the local time and its relationship to GMT on an international time chart.
  2. If both locations are either ahead of GMT (GMT+) or behind GMT (GMT-), subtract the smaller from the larger figure. For example, Toronto is GMT -5 and San Francisco is GMT -8. Subtracting 5 from 8 gives 3, so there is a 3-hour time difference between the two locations.
  3. If the local time is ahead of GMT (GMT+) at one location and behind GMT (GMT -) at the other location, add the figures. For example, New York is GMT -5 and Rome is GMT +1. Adding 5 plus 1 gives 6, so there is a 6-hour time difference between the two locations.

To Calculate the Elapsed Travel Time

  1. Convert departure and arrival times to 24-hour clock time.
  2. Subtract the departure time from the arrival time.
  3. When traveling east, subtract 1 hour for every time zone crossed. When traveling west, add 1 hour for every time zone crossed. The result is the actual travel time.

For example, suppose the departure time from Paris is 12:15 pm and the arrival time in New York is 1:25 pm. Converted to 24-hour clock time, the departure time is 1215 and the arrival time is 1325. The difference is 1 hour and 10 minutes. Because six time zones are crossed going west from Paris to New York, add 6 hours to the result. Thus the elapsed travel time in this case is 7 hours and 10 minutes.


The imaginary grid used for locating places on earth includes:

✓ The North and South Poles.

✓ The equator that separates the globe into Northern and Southern Hemispheres.

✓ Parallels that measure latitude, the distance north or south of the equator.

✓ Meridians that measure longitude, the distance east or west of the prime meridian.

The earth's time zones:

✓ Are measured from the prime meridian in Greenwich, England.

✓ Are 1 hour apart.

✓ Meet in the Pacific at the international date line.