Time Zone

A time zone is a geographic region in which people use the same standard time. This system ensures that cities in the same zone display the same time, rather than each using the local solar time particular to its location.

Time zones are usually based on hourly offsets calculated from the international reference point UTC (Coordinated Universal Time). For example, Somalia uses UTC+3, meaning the time there is three hours ahead of UTC. Some offsets are whole hours, others are half-hours or quarter-hours.

Origins of the system

Before the late nineteenth century, each city kept local solar time, determined by the position of the sun at its highest point in the sky. The expansion of railways and telegraph networks created a need for synchronized time, as travel schedules became difficult when every location operated on its own time. This led to the adoption of the zone system, in which the Earth is divided into sections of uniform time.

Zone boundaries and borders

In theory, Earth can be divided into 24 zones, each spanning 15 degrees of longitude, since the planet rotates 360 degrees in 24 hours. In practice, time zone boundaries are not straight lines: they follow country, regional, and administrative borders so that each state can use a single official time. Some large countries, such as China, use only one official time, while others, such as Russia, span multiple zones.

Daylight saving time

Many countries far from the equator use daylight saving time, advancing clocks by one hour during summer months and reverting them in autumn. The purpose is to make better use of daylight. Countries near the equator, where day and night are roughly equal length throughout the year, typically do not use this practice.

Technology use

Computer and internet systems rely on global databases recording time zones and historical changes made by countries. The use of UTC within these systems enables time data to be compared accurately and then converted to the local time of the user.