Guide
Airport Time Zones Explained
Every airport operates on the local civil time of its city, expressed as an offset from Coordinated Universal Time (UTC). A departure or arrival time on your ticket is always the local clock time at that specific airport, never your home time or the other end of the route. Knowing an airport's UTC offset lets you compare two places directly and calculate true flight duration.
What a UTC offset actually means
UTC is the global reference clock, roughly equivalent to the old Greenwich Mean Time. Each region sets its clocks a fixed number of hours ahead of or behind UTC. Seoul's ICN runs at UTC+9, meaning local time is nine hours ahead of UTC. New York's JFK sits at UTC-5 in winter. London's LHR is the anchor point at UTC+0 during standard time. The offset is written with a plus or minus sign, then hours and sometimes minutes.
Most offsets are whole hours, but not all. A handful of places use half-hour or 45-minute offsets, which surprises many travellers:
| ICN Seoul | UTC+9, no daylight saving |
|---|---|
| DXB Dubai | UTC+4, no daylight saving |
| DEL Delhi | UTC+5:30, a half-hour offset |
| KTM Kathmandu | UTC+5:45, a 45-minute offset |
| JFK New York | UTC-5 winter, UTC-4 summer (DST) |
| SYD Sydney | UTC+10 winter, UTC+11 summer (DST) |
Daylight saving shifts the offset
An airport's offset is not always constant. Regions that observe daylight saving time move their clocks forward by an hour for part of the year, changing the UTC offset seasonally. New York alternates between UTC-5 and UTC-4; London between UTC+0 and UTC+1. Crucially, the northern and southern hemispheres shift in opposite months, and many places (including most of Asia and the Gulf) never change at all. This is why the time difference between two cities can vary by an hour or two depending on the date.
Why flight plans use UTC but tickets use local time
Aviation runs on two clocks for a good reason. Passenger-facing schedules use local time because that is what people plan around: you want to know when to arrive at the airport and when someone should collect you. But pilots, air traffic control, and flight plans all use UTC, spoken as "Zulu" time. A single global reference removes ambiguity when a flight crosses several zones in one leg. A crew filing a plan from LAX to NRT references UTC throughout, then the arrival is converted to Tokyo local time for the display board.
Calculating real flight duration
Because both times on your ticket are local, you cannot simply subtract them. Convert both to UTC first, then subtract. A flight leaving ICN (UTC+9) at 18:00 arrives JFK (UTC-4 in summer) at 19:00 the same calendar day. In UTC that is a 09:00 departure and a 23:00 arrival: a 14-hour flight, even though the clock numbers barely moved. Eastbound and westbound journeys produce very different-looking clock changes for the same physical duration.
Red-eye flights and jet lag
A red-eye is an overnight flight that departs late and arrives early, letting you sleep in transit and land at the start of a working day. Time zones make these possible: a westbound red-eye can arrive at an earlier local clock time than it left. Jet lag comes from your body clock lagging behind local time after crossing several zones. As a rough guide, each time zone crossed takes about a day to adjust to, and eastward travel (shortening your day) is generally harder to recover from than westward.
How this site shows local time
Every airport page on AeroCodes displays the airport's current local time, computed live from its IANA time zone, so the value already accounts for daylight saving on today's date. Instead of memorising whether Sydney is currently UTC+10 or UTC+11, you can open the page for SYD and read the correct local clock directly. Comparing two airport pages side by side gives you the real-time difference between them at this exact moment, which is the fastest way to judge when a call or a landing will actually fall in each city's day.