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Time Difference

Have you ever wondered how time zones work and why we have them?

This will give you a brief overview of the International Date Line, the Greenwich Meridian, and other time-related matters regarding time zones.



What is a Time Zone?

A standard time zone generally refers to any of the 24 regions of the earth’s surface (loosely divided by longitude) in which standard time is kept. However, the number of standard time zones is debatable and discussed among various sources, particularly with regard to the International Date Line. It is important to note that some countries have non-standard time zones, usually with a 30-minute offset (some have a 45-minute offset). For example, India maintains a time zone of five hours and 30 minutes ahead of Coordinated Universal Time (UTC+5:30).

Time zones can be determined by how countries’ and states’ borders are positioned. Individual zone boundaries are not straight because they are adjusted for the convenience and desires of local populations. Moreover, some geographically large countries, such as India and China, use only one time zone but other large countries, such as Russia and the United States, have more than one time zone.

Why We Have Time Zones?

Many towns and cities around the world used to set clocks based on observing the sun and the stars. This occurred prior to the late 19th century. Dawn and dusk occur at different times at different places because of the earth’s rotation. However, time differences between distant locations were barely noticeable because of long travel times and the lack of long-distance communications. The expansion of transport and communications, as well as trade globalization, during the 19th century created a need for a more unified time-keeping system.

How Time Zones Work?

Each time zone is then theoretically 15 degrees wide, corresponding to a one-hour difference in mean solar time. The shape of time zones is changed, in practice, to match internal and international borders. Civil time changes by one hour forward and backward respectively for every 15 degrees east or west of the Greenwich Meridian.

If you travel around the world, changing standard time by one hour each time you enter a new time zone, then a complete circuit would mean that you adjusted your clock or watch time by 24 hours. This would lead to a difference of one day between the date on your clock and the real calendar date. To avoid this, countries are on either side of the International Date Line which runs down the middle of the Pacific Ocean. If you cross the date line moving east, you subtract a day, whereas if you are moving west you add on a day.

The International Date Line

The International Date Line is an imaginary line of longitude on the earth’s surface located at about 180 degrees east (or west) of the Greenwich Meridian. This is the line across which the date changes by one day. It makes some deviations from the 180-degree meridian to avoid dividing countries in two, especially in the Polynesia region. Moreover, the time difference between either sides of the International Date Line is not always exactly 24 hours because of local time zone variations.

The Greenwich Meridian

The Greenwich Meridian, is a north-south line selected as the zero-reference line for astronomical observations. The line in Greenwich, the United Kingdom (UK), represents the world’s prime meridian – longitude zero degrees. Every place on earth is measured in terms of its distance east or west from this line. The line divides the earth’s eastern and western hemispheres just as the equator divides the northern and southern hemispheres.


GMT was first adopted as the world’s time standard at the Washington Meridian Conference in 1884. GMT is no longer the basis for civil time but is now loosely interchanged with UTC to refer to time kept on the Greenwich meridian (longitude zero). Places such as the United Kingdom observe GMT during the non-daylight saving period. UTC is the basis for civil time in many places worldwide.

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