Timezones

Time zones are geographic regions in which the local standard time is the same. They are defined by offsets from a reference time, typically Coordinated Universal Time (UTC), and are used to standardize clocks within a region so that daily activities align with daylight. The borders between time zones are often irregular, reflecting political boundaries, population centers, and practical considerations such as commerce and travel.

Historically, standard time arose in the 19th century with rapid railway expansion and the need for consistent schedules. In 1884 the International Meridian Conference established the prime meridian at Greenwich and recommended a system of time based on UTC, with Greenwich Mean Time serving as a practical reference for many years.

Modern offsets from UTC can be written as UTC±H:MM, and they range from UTC−12 to UTC+14. Most zones use whole-hour offsets, but some use fractional hours or minutes, including UTC+05:30 in India and UTC+05:45 in Nepal. Daylight saving time, when observed, shifts clocks by one hour for part of the year; practice and dates vary by country and region.

Political boundaries influence time zones, resulting in anomalies such as countries adopting single time zones across large areas or administrative changes that compress or expand the number of zones. Examples include China’s national use of a single time zone despite its size, and periodic reforms in Russia and elsewhere. DST is not universally observed and its adoption can change.

Time zone data underpin software and scheduling systems. The IANA Time Zone Database and other resources track historical changes to offsets and DST rules, enabling accurate time conversions, calendar calculations, and travel planning across different regions.