A complete introduction to how the world's time zones work — from UTC offsets and daylight saving time to the IANA tz database that powers every phone, computer, and online service.
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What a time zone really is
A time zone is a geographic region that shares a single standard time. The Earth was originally split into 24 one-hour strips by longitude (15° per hour), but modern boundaries follow political borders. Some zones are offset by 30 or 45 minutes, and countries as wide as China or India choose a single zone for their entire territory.
UTC: the global anchor
UTC (Coordinated Universal Time) is the reference point every other zone is measured against. It replaced GMT in 1972 and is kept by an international network of atomic clocks. You'll see offsets written as UTC+09:00 (Tokyo), UTC−05:00 (New York in winter), or UTC+05:30 (India — a half-hour zone). UTC itself never shifts for daylight saving.
How Daylight Saving Time (DST) works
DST moves the clock forward one hour in spring and back in fall so evenings get more daylight. Most of North America and Europe observe it; most of Asia and Africa do not. During DST a zone's offset changes — New York moves from UTC−5 to UTC−4, London from UTC+0 to UTC+1. Start and end dates differ by country and can change year to year.
The IANA tz database
The IANA Time Zone Database (also called tz or zoneinfo) is the authoritative source of timezone rules and is used by iOS, Android, Linux, macOS, Java, Python, and Postgres. Zones are named Region/City — America/New_York, Asia/Taipei, Europe/London. Always prefer these names over abbreviations like EST or IST, which are ambiguous (IST can mean India, Ireland, or Israel Standard Time).
Best practices for scheduling and development
Store timestamps in UTC and convert only when displaying to users. For meetings, pin a fixed UTC time and let each participant's device render local time. Around DST transitions, watch for skipped or repeated hours. Use our Timezone Converter for quick lookups, and check the World Time Zone List to scan every common zone and its UTC offset at a glance.
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Timezone Converter
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World Time Zone List
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Questions fréquentes
Q: Why does China use only one time zone when it spans five?
A: Since 1949 China has used a single political time zone (UTC+8, "Beijing Time") for national cohesion, even though its western regions geographically fall in UTC+5 or UTC+6. Xinjiang unofficially operates on UTC+6.
Q: Are UTC offsets always whole hours?
A: No. Several zones use 30- or 45-minute offsets: India (UTC+5:30), Iran (UTC+3:30), Nepal (UTC+5:45), parts of Australia (UTC+9:30, UTC+8:45). A few historical zones had offsets precise to the second.
Q: What happens to clocks during a DST "spring forward"?
A: The hour from 2:00 AM to 3:00 AM is skipped — it never exists on that calendar date. In fall, the 1:00–2:00 AM hour repeats. Code that assumes every day is exactly 24 hours long will break around these transitions; always use a battle-tested library.