Time Zones Explained
Time zones explained — 38 distinct UTC offsets (not 24), India +05:30 and Nepal +05:45, the IANA tz database, and why offsets follow politics, not longitude lines.
By Steve K.. Published . Last updated .
A time zone is a region that shares a common civil time, defined as a fixed offset from Coordinated Universal Time (UTC). About 38 distinct standard-time offsets are in active use today, including 30-minute (India +05:30) and 45-minute (Nepal +05:45) offsets — boundaries follow politics, not longitude.
Time zones are the bridge between solar time (which depends on longitude — see /learn/what-is-longitude) and civil time (which is administratively assigned by governments). The prime meridian that anchors UTC lives at /learn/the-prime-meridian. The naive picture — 24 zones, each 15° wide, clean UTC offsets from −12 to +12 — is almost completely wrong in operational practice. There are 38+ distinct offsets, non-hour increments are common, DST adds another layer of complexity, and the boundaries follow political not longitudinal lines. This article runs the structure of the modern time-zone system, the IANA database that captures it, the relationship between time zones and longitude, the half-hour and 45-minute offsets that complicate everything, and the operational implications for software. The companion pillar /learn/what-is-longitude covers the longitude side of the equation; supports like /learn/utc-explained and /learn/leap-seconds-explained go deeper.
The structure: time zones are political, not geographic
The "24 time zones, each 15° wide" model from school is the idealisation. The reality is that every time zone follows political boundaries set by the local government. China uses one time zone for the whole country (~5 solar-time hours wide); Russia uses 11. France has 12 time zones counting its overseas territories.
| Region | Time zone(s) | Solar-time span | Why this configuration |
|---|---|---|---|
| China (mainland) | 1 (UTC+08:00) | ~5 hours solar | Single zone since 1949 for political unity |
| Russia | 11 | ~10 hours solar | Spans Europe to the Pacific |
| USA + dependencies | 9 zones (HST, AKST, PST, MST, CST, EST, AST, plus Samoa, Guam) | ~7 hours solar | Continental spread + Pacific territories |
| India | 1 (IST = UTC+05:30) | ~1.5 hours solar | Single zone since 1947; 30-minute offset is unusual |
| Australia | 5 (AWST, ACWST, ACST, AEST, AEDT) | ~3 hours solar | Includes one 45-minute offset (ACWST = +08:45 in part of WA) |
| France (incl. overseas) | 12 | Spans 5 continents | Most time zones of any country |
| Antarctica | No standard | All | Bases run on the time zone of their home country |
The result: the number of time zones globally is not 24 but is
governed by how many distinct combinations of (offset, DST rule) are in
use. The IANA tzdata project enumerates them; current count is several
hundred identifiers (e.g. America/New_York, Europe/London,
Asia/Tokyo) covering ~38 distinct standard-time offsets.
The 15° rule and where it breaks
Earth rotates 360° in ~24 hours, so each 15° of longitude corresponds to 1 hour of mean solar time difference. This is the theoretical basis for time zones — but the practical boundaries depart from it consistently.
| Longitude band | Theoretical UTC offset (15° rule) | Actual offset for cities in this band | Departure from theory |
|---|---|---|---|
| 7.5° W to 7.5° E | UTC+00:00 | UTC+00:00 (UK, Portugal, west Africa); UTC+01:00 (most of W. Europe) | Most of W. Europe is one hour ahead of solar |
| 37.5° E to 52.5° E | UTC+03:00 | UTC+03:00 (Moscow, Iraq); UTC+03:30 (Iran) | Iran uses 30-minute offset |
| 67.5° E to 82.5° E | UTC+05:00 | UTC+05:30 (India); UTC+05:45 (Nepal); UTC+05:00 (Pakistan) | Multiple non-hour offsets in one band |
| 97.5° E to 112.5° E | UTC+07:00 | UTC+08:00 (China, all of it) | China + 1 hour vs solar over its whole width |
| 127.5° E to 142.5° E | UTC+09:00 | UTC+09:00 (Japan, Korea); UTC+09:30 (most of central Australia) | Australian central states use 30-min offset |
| 172.5° E to 172.5° W (antimeridian) | UTC±12:00 | UTC+12:45 (Chatham Is.); UTC+13:00 (Samoa); UTC+14:00 (Kiribati Line Is.) | Three different offsets all near the antimeridian |
Western Europe is the canonical example of "time zone wider than 15° band": France, Spain, Portugal and the UK all sit at similar longitudes but use different offsets — Spain at UTC+01:00 is an hour ahead of solar (residents see noon at ~13:30 civil time in Madrid).
The non-hour offsets
The "everyone uses whole-hour offsets" assumption is wrong. The following countries / regions use 30-minute or 45-minute offsets, and they include several of the world's most populous nations.
| Region | UTC offset | Population (~M) | Adopted | Historical reason |
|---|---|---|---|---|
| India | +05:30 | ~1,420 | 1947 | Compromise between two extreme longitudes within the country |
| Sri Lanka | +05:30 | ~22 | 1996 (reverted) | Tracks India for trade reasons |
| Nepal | +05:45 | ~30 | 1986 | Distinct from India by 15 min for political identity |
| Iran | +03:30 | ~88 | 1969 | Mean solar time of Tehran rounded |
| Afghanistan | +04:30 | ~40 | pre-1944 | Mean solar time of Kabul |
| Myanmar | +06:30 | ~54 | 1920 | Mean solar time of central Myanmar |
| Australia (central states) | +09:30 / +10:30 DST | ~2 | 1899 | ACST: South Australia, Northern Territory |
| Australia (Chatham, NZ) | +12:45 / +13:45 DST | ~0.0007 | 1957 | Chatham Islands east of NZ |
| Australia (Eucla, WA) | +08:45 | A few hundred | unofficial | Eucla and surrounding desert; not officially observed by federal govt |
| Newfoundland (Canada) | −03:30 / −02:30 DST | ~0.5 | pre-1935 | Distinct from Atlantic Canada by 30 minutes |
| Venezuela | −04:30 (2007-2016) → −04:00 (current) | ~28 | 2007-2016 only | Chávez introduced; reverted by Maduro for "economic alignment" |
| North Korea | +08:30 (2015-2018) → +09:00 (current) | ~26 | 2015-2018 only | Reverted in 2018 to align with South Korea for diplomatic reasons |
The 30-minute and 45-minute offsets are not historical quirks; they serve ~1.6 billion people today (mostly India + Iran). Any international software that assumes whole-hour offsets is broken in those regions.
The IANA Time Zone Database
The Internet Assigned Numbers Authority maintains the canonical database of every time zone in the world, every DST rule, and every historical change since 1970 (with some pre-1970 data on a best-effort basis).
| IANA component | What it stores | Example |
|---|---|---|
| Identifier | Region / city name (Continent/City) | America/New_York, Europe/London, Asia/Kolkata |
| Standard-time offset | Constant offset from UTC for the zone's standard time | America/New_York: UTC−05:00 (EST) |
| DST rule | When DST starts/ends and the offset during DST | America/New_York: spring forward 2nd Sun March, fall back 1st Sun November; DST offset is UTC−04:00 (EDT) |
| Historical zoneinfo | Every offset/rule change since (typically) 1970 | America/New_York used UTC−05:00 with various DST rules dating to 1883 |
| Aliases | Old or deprecated names that still work | US/Eastern → America/New_York; GMT → Etc/GMT |
| Etc/* zones | Synthetic fixed-offset zones for places without a city | Etc/UTC, Etc/GMT+5, Etc/GMT-8 |
Maintained by Theodore Ts'o since 2005 (after Arthur David Olson
retired); released several times a year as governments change their
DST rules. The data lives in a plain-text format (zone1970.tab,
southamerica, etc.) that's parsed by zoneinfo files in every
Unix system and the JavaScript Intl.DateTimeFormat API in every
modern browser.
Daylight Saving Time
About 70 countries observe some form of seasonal time adjustment, typically advancing clocks 1 hour in spring and reverting in autumn. The rules vary dramatically.
| Region | DST in use? | Start / end (typical) | DST offset | Notes |
|---|---|---|---|---|
| United States (most states) | Yes | 2nd Sun Mar / 1st Sun Nov | +1 hour | Arizona and Hawaii do not observe; some movement to abolish |
| European Union | Yes | Last Sun Mar / Last Sun Oct | +1 hour | Switzerland & UK follow EU schedule; EU debating abolition 2024+ |
| UK | Yes | Last Sun Mar / Last Sun Oct | +1 hour | British Summer Time (BST = UTC+01:00) |
| Australia (states observing) | Yes | 1st Sun Oct / 1st Sun Apr (Southern Hemisphere reversed) | +1 hour | NSW, VIC, SA, TAS, ACT observe; QLD, WA, NT do not |
| Brazil | No (abolished 2019) | — | — | Previously observed; now permanent standard time |
| Russia | No (abolished 2011) | — | — | Permanent UTC+03 (Moscow Time) |
| China | No | — | — | Never observed in mainland |
| India | No | — | — | Single year-round IST |
| Japan | No | — | — | Never observed |
| Egypt | Yes (reinstated 2023) | Last Fri Apr / Last Thu Oct | +1 hour | Reinstated after 2014 abolition |
DST is a moving target. Egypt abolished DST in 2014, observed it again from 2023; Brazil dropped DST in 2019; Russia abolished in 2011; the EU has been debating abolition since 2018 without finalisation. Software cannot guess — it must look up current rules in the IANA database, which is updated for each change.
How time zones interact with longitude
Time zones approximately track longitude but never perfectly. The relationship has three consistent patterns.
| Phenomenon | Mechanism | Example |
|---|---|---|
| Solar noon vs civil noon | A location at the exact centre of its time zone has solar noon at civil noon; offset locations diverge | Madrid solar noon ~13:31 civil time |
| Time zone wider than ideal | Some countries cover several solar hours but use one zone | China: UTC+08:00 covers ~5 solar hours; far-western noon at ~14:00 |
| Wrong side of antimeridian | Kiribati moved its IDL east of its territory in 1995 | Line Islands at UTC+14:00 (east-most time on Earth) |
| Country-specific 30/45-min offset | Adopted for political distinctness or to centre solar time better | India, Iran, Afghanistan, Nepal, parts of Australia |
| Politically motivated change | Country switches zones for political or economic reasons | North Korea reverted from UTC+08:30 to +09:00 in 2018; Venezuela 2007-2016 |
The cumulative effect: longitude is a suggestion for civil time, not a determinant. A web application that converts coordinates to civil time must use a geographic time-zone polygon database (such as timezone-boundary-builder, which generates polygons compatible with IANA identifiers).
Time-zone software pitfalls
| Pitfall | Why it happens | Mitigation |
|---|---|---|
| Storing local time without offset | User enters "9:00 AM" without time zone | Always store IANA identifier alongside; or store as UTC + IANA tz for display |
| DST transition ambiguity | Same local time appears twice (fall back) or never (spring forward) | Use date-fns-tz, Luxon, Temporal, or equivalent that handles transitions |
| Historical rule changes | IANA database back-dated changes recurring meetings | Recompute meeting times when tzdata updates; don't cache offsets indefinitely |
| Hardcoded offset (UTC−05:00 instead of America/New_York) | Fails on DST transitions | Always use IANA identifiers in storage |
| Server time zone different from user | Server in UTC, user in EST | Always work in UTC server-side; format with user's tz at display time |
| JavaScript Date timezone confusion | new Date() uses system tz; .toISOString() is UTC | Use Intl.DateTimeFormat or a tz library; never assume system tz |
| Date-vs-time-with-zone confusion | "April 15, 2026" without time is ambiguous globally | For dates, specify the relevant time zone (calendar date is local; ISO 8601 date is UTC) |
The Temporal API (TC39 stage 4, shipping in browsers and Node 22+) is
the modern JavaScript answer to date/time handling: it requires every
date-time value to declare its time zone (or explicitly mark as
"floating"). The old Date object's "implicit local time"
behaviour is the root of most time-zone bugs.
Common misconceptions
Related pillars
The other seven pillar concepts on Coordinately:
- What is latitude and longitude? — the spatial reference time zones offset from
- Coordinate formats explained — coordinate notations that complement time-zone metadata
- How GPS works — GPS time, the atomic-clock reference behind UTC
- What is a map projection? — world maps that show time-zone bands
- What is a geodetic datum? — the spatial reference frame underneath time geography
- History of latitude and longitude — the 1884 conference that codified time zones
- Great-circle distance — distance between two places at different local times
Related
- The 1884 International Meridian Conference— The diplomatic origin of the Greenwich-based time-zone system
- What Is Longitude?— The geographic basis of time zones — 15° per hour
- A History of Latitude and Longitude— The broader historical arc
- The Longitude Problem— Why timekeeping became central to navigation
- Methodology— How content is sourced and verified
Frequently asked questions
How many time zones are there?
About 38 distinct civil-time offsets are currently in use worldwide — substantially more than the often-cited 24. The 24-zone figure assumes integer-hour offsets only; the real world includes non-integer offsets like India (+5:30), Nepal (+5:45), Iran (+3:30), Newfoundland (-3:30), Afghanistan (+4:30), Marquesas (-9:30), Chatham Islands (+12:45), and several others. Counting all DST-shifted variants pushes the number higher; the IANA tz database tracks ~600 named zones including historical aliases. The widest current range is from UTC-12 (uninhabited Baker Island) to UTC+14 (Kiribati Line Islands), spanning 26 hours.
What is the IANA tz database?
The IANA Time Zone Database (also called the tz database or Olson database) is the authoritative reference for civil time zone information used worldwide. It was created in 1986 by Arthur David Olson at the U.S. National Institutes of Health and is currently coordinated by Paul Eggert at UCLA. It tracks ~600 named zones using a Region/City naming convention (America/New_York, Europe/Paris, Asia/Tokyo, Pacific/Auckland). The database covers current civil time, historical DST transitions, leap seconds, and timezone abolitions back to 1970 (with partial pre-1970 coverage). Every Unix-like operating system, Java JVM, Python, Node.js, .NET, Go, Ruby, and most other major platforms use the tz database as their time-zone authority.
Why are some time zones not whole hours?
Several reasons: (1) Solar-noon optimization — a country with a small east-west extent may pick a half-hour offset to keep solar noon close to clock noon, as India did at +5:30. (2) Historical compromise — Newfoundland's -3:30 reflects its astronomical longitude (52°W ≈ -3:30 from Greenwich) and was kept when neighboring zones standardized to whole hours. (3) Political independence — Nepal at +5:45 (15 minutes ahead of India) explicitly distinguishes itself from India. (4) Specific events — Iran adopted +3:30 in 1935 to match a then-popular Tehran solar-noon convention. The non-integer offsets are stable and not unusual; they're a normal feature of the world's time-zone landscape.
What is UTC and how does it relate to time zones?
UTC (Coordinated Universal Time) is the world's primary time reference, defined by ITU-R Recommendation TF.460-6 and coordinated by the Bureau International des Poids et Mesures (BIPM). UTC is computed from approximately 400 atomic clocks at ~80 laboratories in ~50 countries, with leap seconds added occasionally to keep it within 0.9 seconds of UT1 (Earth-rotation-based time). Every civil time zone is defined as an offset from UTC. The U.S. East Coast in winter is UTC-5 (Eastern Standard Time); in summer it's UTC-4 (Eastern Daylight Time). The offset is the time zone; UTC is the reference everything is offset from.
How should software handle time zones?
The standard pattern: store every timestamp internally as UTC; convert to local time only at presentation. This avoids the most common bugs — assuming local time, storing only an offset without the IANA zone name (an offset doesn't tell you the historical DST behavior), and breakage at DST transitions (some local times happen twice; some don't happen at all). Use IANA zone names like 'America/New_York' rather than ambiguous abbreviations like 'EST' (which can mean Eastern Standard Time or Australian Eastern Standard Time). For wire formats and storage, use ISO 8601 / RFC 3339 with explicit UTC offset (e.g., '2026-05-24T17:30:00+00:00' or the 'Z' suffix). For database columns, prefer 'TIMESTAMP WITH TIME ZONE' (PostgreSQL) or always-UTC storage rather than naive local timestamps.
How many time zones are there in the world?
About 38 distinct standard-time offsets are in active use today — far more than the textbook "24 zones, each 15° wide" model. The IANA Time Zone Database lists ~600 named zones, because each region with a distinct historical DST rule gets its own identifier. Offsets range from UTC−12:00 (Baker Island, uninhabited) to UTC+14:00 (Kiribati Line Islands). Non-integer offsets include India +05:30, Nepal +05:45, and Newfoundland −03:30.
Does China have only one time zone?
Yes — mainland China observes a single time zone, China Standard Time (UTC+08:00), since 1949 for political unity. This covers roughly 5 hours of solar time east-to-west; far-western Xinjiang sees solar noon at around 14:00 civil time. Hong Kong, Macau and Taiwan also follow UTC+08:00. Some local communities in Xinjiang informally observe an unofficial "Xinjiang Time" at UTC+06:00 that better matches solar noon, but it has no legal standing.
When does Daylight Saving Time start and end?
In the United States, DST runs from the second Sunday of March to the first Sunday of November (Arizona and Hawaii do not observe). The European Union runs DST from the last Sunday of March to the last Sunday of October. Most southern-hemisphere DST-observing countries (Australian states, Chile, Paraguay, NZ) reverse this — their DST runs October to April. About 70 countries observe some form of DST; many have abolished it (Russia 2011, Brazil 2019), and the EU has debated abolition since 2018.
Sources
- IANA — IANA Time Zone Database (tz database) — maintained by Paul Eggert · https://www.iana.org/time-zones · Accessed .
- BIPM — Bureau International des Poids et Mesures — UTC and the world atomic time · https://www.bipm.org/en/time-ftp/utc · Accessed .
- NIST — NIST Time & Frequency Division — UTC and time zones · https://www.nist.gov/pml/time-and-frequency-division · Accessed .
- ITU — ITU-R Recommendation TF.460-6 — Standard-frequency and time-signal emissions · https://www.itu.int/rec/R-REC-TF.460/en · Accessed .
Cite this article
APA format:
Steve K. (2026). Time Zones Explained. Coordinately. https://coordinately.org/learn/time-zones-explained
BibTeX:
@misc{coordinately_timezonesexplained_2026,
author = {K., Steve},
title = {Time Zones Explained},
year = {2026},
publisher = {Coordinately},
url = {https://coordinately.org/learn/time-zones-explained},
note = {Accessed: 2026-06-05}
}