Why time-zone maths trips travellers up (and how to get it right)

Time zones look like simple arithmetic until you do it at 4 a.m. in a transit lounge. The rules aren’t hard, but there are more exceptions than people expect, and the exceptions are exactly where missed calls, missed connections and “wait, what day is it?” come from. These are the questions we get most, in roughly the order people hit them.

Why isn’t the offset always a whole hour?

Because time zones are political, not geometric. In theory the world splits into 24 slices of 15° longitude, each one hour apart. In practice, each government picks the offset from UTC that suits its geography, its trading partners and its history. Most pick whole hours. Several don’t:

• India uses UTC+5:30 for the whole country (timeanddate).
• Nepal uses UTC+5:45, fifteen minutes ahead of India (timeanddate).
• Iran uses +3:30, Newfoundland −3:30 and the Marquesas Islands −9:30.
• The Chatham Islands of New Zealand use UTC+12:45, and +13:45 in their summer (timeanddate).
• Eucla, a strip of Western Australia along the Eyre Highway, unofficially keeps +8:45. It is common enough that the tz database gives it its own entry, Australia/Eucla.

A practical consequence: if you set a meeting between London and Kathmandu with a mental “they’re five or six hours ahead”, you’ll be 15 minutes out either way. Always look the offset up.

Why did my flight arrive “before” it left?

Because departure and arrival times are printed in local time at each end. If you fly east to west across a lot of zones, or across the International Date Line, the clock at the destination can read earlier than the clock you left.

Take a worked example with round numbers. New Zealand’s summer time starts on the last Sunday of September. In late September 2026 Auckland is on NZDT (UTC+13) and Honolulu is on HST (UTC−10), all year, with no DST.

1. You leave Auckland at 19:00 on Tuesday, local time. That’s 06:00 Tuesday UTC.
2. AKL–HNL is about 7,090 km great-circle. Our site’s block-time estimate is roughly 9 hours; real schedules vary, so check the airline.
3. Add 9 hours and you land at 15:00 Tuesday UTC.
4. In Honolulu, 15:00 UTC is 05:00 on Tuesday, fourteen hours “before” you took off.

Nothing strange happened to you. You just flew across the line where the calendar date changes. It works the other way too: fly west across the Pacific and you can lose most of a calendar day. Countries can even move themselves across the line. Samoa skipped Friday 30 December 2011 entirely when it switched sides to line up with Australia and New Zealand.

Rule of thumb: to get a real flight duration, convert both ends to UTC. Never subtract local times.

What happens on DST changeover days?

Three things catch people out.

1. Countries change on different dates. The EU and UK move clocks at 01:00 UTC on the last Sunday of March and October (Directive 2000/84/EC). The US moves on the second Sunday of March and the first Sunday of November (NIST). This autumn, Europe falls back on 25 October 2026 and the US on 1 November 2026. For that one week, New York is four hours behind London instead of five. Recurring transatlantic meetings drift by an hour, and someone always misses one.

2. The southern hemisphere runs the other way. When Europe springs forward, Australia and New Zealand fall back, and vice versa. The gap between London and Sydney swings between 9, 10 and 11 hours over a year. Many places near the equator, including most of Africa, much of Asia and all of Japan, don’t use DST at all.

3. Some local times don’t exist, and some happen twice. On spring-forward night, UK clocks jump from 01:00 straight to 02:00, and Central European clocks from 02:00 to 03:00. A 01:30 alarm in London, or a 02:30 departure in Paris, refers to a time that never happens that night. On fall-back night, an hour repeats: 01:30 happens twice in London. Airlines and railways know this and schedule around it. Hotel wake-up calls, phone alarms set on a laptop, and taxi bookings sometimes don’t. On changeover nights, set times explicitly and double-check anything early the next morning.

Special mention for Lord Howe Island, Australia, which moves its clocks by only 30 minutes for DST. Proof that there is always one more exception.

Why do some countries have one zone despite spanning several?

China is the famous case. The country spans roughly five geographical time zones, but it has officially run on a single time, Beijing Time (UTC+8), since 1949 (Time in China; timeanddate). In the far west, in Xinjiang, the sun can rise very late by the clock. Many locals also use an unofficial “Xinjiang time” two hours behind Beijing for daily life. Always ask which one a bus timetable, shop sign or meeting invitation means.

India is a smaller version of the same thing: one national offset, UTC+5:30, across a wide country, so sunrise in the north-east comes much earlier by the clock than in Gujarat. The opposite case is also common. The US, Canada, Russia, Brazil and Australia split into several zones, sometimes along lines that zigzag around county or state borders. In Australia, states in the same longitude band don’t necessarily agree on DST: Queensland stays on standard time while New South Wales changes.

For travellers the lesson is the same: a time zone is a rule attached to a place, not a band on a map.

How does TravellerSeed calculate it?

We don’t store fixed offsets like “+1” or “+5:30”. Every airport in our dataset is tagged with an IANA time-zone identifier, such as Europe/London, Asia/Kathmandu or Pacific/Chatham. You can see these on any airport page. The IANA tz database is the shared reference maintained by volunteers and used by operating systems, phones and browsers. It records each place’s full history of offset and DST changes, and it’s updated several times a year when governments change their rules. The tz-announce mailing list is where the changes are announced.

When you look at a route or airport page, your browser does the conversion with the JavaScript Intl.DateTimeFormat API. We pass it the IANA zone and the date, and your browser applies the rules for that exact date. That has two consequences:

• DST is handled automatically for the date in question. A route page viewed in September and one viewed in November will show different time differences if one end changes its clocks in between. That’s correct, not a bug.
• Accuracy depends on your browser’s copy of tz data. Modern browsers update it with normal releases. If a government changes its rules at short notice, an out-of-date browser can be wrong for a while. For anything important, such as a visa-entry deadline or an early check-in, cross-check with timeanddate.com or the airline’s own itinerary.

Flight durations we show are great-circle estimates (distance ÷ 800 km/h), not schedules. Real block times include taxiing, routing and winds, so they are longer. Use our number to get a feel for the trip, and the airline’s for your connections.

A short checklist for your next trip

1. Write down both UTC offsets for your travel date, not today’s.
2. Convert to UTC before you subtract anything.
3. Check whether either end changes clocks during your trip, especially in late March, late October and early November.
4. Ask what time a local timetable uses, particularly in western China.
5. Set your phone to update its time zone automatically, but keep one clock on home time so you know when to call.