Prouči

In the old days each town would keep their own times, with a town clock being set to noon when the Sun reached its highest point in the sky. However, by the late 19th century rail had begun to move people across great distances. Schedules became very confusing as each stop was based on a different town's time. Thus, a need emerged to standardise time so rail could operate more efficiently.

In 1884 delegates from 26 countries met in Washington DC to agree on a Prime Meridian as a common zero for longitude. The result was Greenwich, England being selected as the international standard for zero degrees longitude and established the 24 time zones.

Today most countries follow the hourly deviations, though they do not always align with longitudes for reasons such as keeping the entire country in one time zone or altering times at certain times of the year, for example, daylight savings time.

Time zones are based on the fact that the Earth moves 15 degrees longitude each hour. Since there are 24 hours in a day, there are 24 standard time zones.

Time zones are counted from the Prime Meridian which is zero degrees longitude. Each time zone is counted at 15 degree intervals and extends 7 and a half degrees either side of a central meridian. For example, Sydney, Australia lies on the 150 degrees east central meridian, and the time zone includes all locations between 142 and a half degrees east and 157 and a half degrees east.

Now I will show you how to calculate time zones.

First, determine whether the time zone is in the eastern or western hemisphere.

If the time zone is east of the Prime Meridian, such as Australia, it will be at an earlier time than Greenwich Mean Time or GMT. If it is west of the Prime Meridian, such as Canada, it will be a later time.

Second, count how many time zones away from Greenwich the location is.

For example, Greenland is one time zone to the west of Greenwich so I will subtract one hour from GMT to determine the time there. Oslo in Norway is one time zone to the east of Greenwich, so I will add just one hour to the current Greenwich Mean Time.

When counting the number of time zones away a location is, count the time zone your location is in, but do not count the time zone that Greenwich is in.

For example, I will count for Buenos Aires, Argentina.

One, two, three, four.

So if the time is 4:34 GMT, I will subtract four hours to find out the time for Buenos Aires. The time will be 0:34.

The International Date Line is an imaginary line, lying 180 degrees in the line of longitude on the opposite side of the Earth to the Prime Meridian. This line separates one calendar day from another. The International Date Line deviates in places to avoid crossing any land.

In 1994 Kiribati shifted the dateline 2,000 miles east to keep the entire country in the eastern hemisphere.

In 2011 Samoa and Tokelau skipped the day and moved westward to be in the eastern hemisphere so they could have the same date as Australia and New Zealand, their main trading partners.