World Time Zones

A time zone is a region of the Earth where people and governments use the same official time. Zones are defined by their difference from UTC (Coordinated Universal Time), for example UTC+3 or UTC−5.

The concept of time zones arose from the need for railways, telegraphs, and ships to operate on a common schedule. Before this, each city used its own local solar time, which made shared scheduling difficult.

Geographic basis

The Earth rotates 360 degrees in a day, equivalent to 15 degrees per hour. The standard system therefore divides the world into 24 zones, each spanning 15 degrees of longitude. Zone boundaries are measured from the Greenwich meridian (0 degrees).

Actual boundaries

Time zone boundaries do not follow straight lines. Governments define them according to regional, national, and commercial considerations. Some large countries use a single zone, while others use several. Some zones also differ by half an hour or quarter hour rather than whole hour increments.

Daylight saving time

Many countries advance their clocks during summer months (daylight saving time) to make better use of natural daylight. This means a country's UTC offset changes throughout the year. Countries near the equator generally do not use it, because day and night length varies little.

International Date Line

Near the 180-degree meridian lies the International Date Line. When crossed, the date advances or decreases by one day. The line is also irregular, deviating around islands to avoid dividing countries.