How Timezones Work

A timezone is a region of the Earth that uses the same standard time. The world is divided into 24 main timezone zones, each roughly 15 degrees of longitude apart, corresponding to one hour of time difference. Timezones exist because the Earth rotates 360° in 24 hours — that's 15° per hour. As you move east, clocks are set forward; as you move west, clocks are set back.

UTC (Coordinated Universal Time) is the primary time standard by which the world regulates clocks. It does not observe daylight saving time and is effectively the same as GMT at the Greenwich Meridian (0° longitude) in London. All timezones are defined as offsets from UTC: • UTC-5 = 5 hours behind UTC (e.g. New York in winter) • UTC+1 = 1 hour ahead of UTC (e.g. Paris in winter) • UTC+5:30 = 5 hours and 30 minutes ahead (e.g. India) UTC replaced GMT as the world standard in 1972, though they remain practically identical for everyday use.

GMT (Greenwich Mean Time) is the time at the Royal Observatory in Greenwich, London (0° longitude). It was established in 1884 as the world's time standard. GMT is still used in the UK during winter. In summer, the UK switches to BST (British Summer Time = GMT+1). The key difference: GMT is a timezone, UTC is a time standard. For everyday purposes they're the same, but UTC is more precise and never changes for DST.

Many countries shift their clocks forward 1 hour in spring and back 1 hour in autumn to make better use of daylight. This is called Daylight Saving Time (DST). • USA: Clocks spring forward 2nd Sunday of March, fall back 1st Sunday of November • EU: Last Sunday of March / October • Australia: First Sunday of October / April (opposite hemisphere) Not all countries observe DST — including China, Japan, India, most of Africa, and Iceland.

The International Date Line (IDL) runs roughly along the 180° meridian in the Pacific Ocean. When you cross it heading east, you subtract one day; heading west, you add one day. This is why countries like Samoa (UTC+13) and New Zealand (UTC+12) can be a full day ahead of countries like Hawaii (UTC-10) — even though they're geographically close.

Most timezones differ by whole hours, but some have unusual offsets: • India: UTC+5:30 • Nepal: UTC+5:45 • Iran: UTC+3:30 • Sri Lanka: UTC+5:30 • Lord Howe Island (AU): UTC+10:30 / +11 These exist for geographic, political, or historical reasons, and make scheduling across certain borders more complex.