A time zone is a region of the globe where the same standard time is used. The Earth is divided into 24 primary time zones, each offset by one hour from its neighbor. Time zones exist so that noon roughly corresponds to when the sun is highest in the sky at any given location.
The Earth rotates 360 degrees in 24 hours, which means it rotates 15 degrees per hour. Each time zone covers approximately 15 degrees of longitude. The system is anchored to the Prime Meridian (0° longitude) in Greenwich, England, where Coordinated Universal Time (UTC) is defined.
Moving east from the Prime Meridian, each zone adds one hour. Moving west, each zone subtracts one hour. So a city at UTC+3 is 3 hours ahead of UTC, while a city at UTC-5 is 5 hours behind.
Most time zones have a "standard" offset that applies year-round. However, many countries shift their clocks forward by one hour during warmer months. This practice is called daylight saving time (DST). During DST, a region's effective UTC offset changes temporarily.
For example, New York uses Eastern Standard Time (UTC-5) in winter but switches to Eastern Daylight Time (UTC-4) from March to November.
Before time zones, every town set its clocks based on the local position of the sun. This worked fine when travel was slow, but became a serious problem when railways and telegraphs connected distant cities in the 19th century.
Train schedules were nearly impossible to coordinate when every station used a different local time. In 1884, delegates from 25 countries met at the International Meridian Conference in Washington, D.C., and agreed to divide the world into standardized time zones based on the Prime Meridian.
Not every time zone follows the clean one-hour-per-zone rule. Several countries and territories use half-hour or even quarter-hour offsets:
Some large countries use a single time zone for political unity. China spans five geographical time zones but operates entirely on UTC+8 (Beijing Time).