Time Zone Math Without the Headache

By WhiteCalc · 2026-11-14

Start with UTC

The easiest way to handle time zones is to stop converting from one city to another directly. Instead, use one common reference: UTC, short for Coordinated Universal Time. Every time zone is described as an offset from UTC, written as UTC+9, UTC−5 and so on. The offset tells you how many hours (and sometimes minutes) the local clock is ahead of or behind UTC.

Converting between two places then takes two small steps:

Convert the local time to UTC by subtracting the offset.
Convert the UTC time to the target place by adding its offset.

Or in one go: target time = source time − source offset + target offset. Because the formula is just addition and subtraction, you can check it with the Time calculator, which also handles the carry over midnight.

Offsets for some common cities

City	Standard time	Summer (DST) time
Seoul	UTC+9	No daylight saving
Tokyo	UTC+9	No daylight saving
London	UTC+0 (GMT)	UTC+1 (BST)
New York	UTC−5 (EST)	UTC−4 (EDT)
Los Angeles	UTC−8 (PST)	UTC−7 (PDT)
Mumbai	UTC+5:30	No daylight saving

Note that India uses a half-hour offset, and a few places (such as Nepal at UTC+5:45) use 45 minutes. Time zones do not always step in whole hours, so always keep the minutes in your calculation.

Worked examples with a single clock reading

Suppose it is 14:00 in Seoul on 15 January. What time is it elsewhere?

UTC: 14:00 − 9 hours = 05:00 on 15 January.
Tokyo: same offset as Seoul, so 14:00.
London: UTC+0 in January, so 05:00 on 15 January.
New York: UTC−5 in January, so 05:00 − 5 hours = 00:00, midnight at the start of 15 January.
Los Angeles: UTC−8, so 05:00 − 8 hours = −3:00, which is 21:00 on the previous day, 14 January.

The same 14:00 in Seoul in July gives a slightly different picture because of daylight saving: London is on UTC+1, so it is 06:00 there, New York on UTC−4 shows 01:00, and Los Angeles on UTC−7 shows 22:00 on 14 July.

A good rule of thumb for East Asia and Western Europe: Seoul and Tokyo are 9 hours ahead of London in winter and 8 hours ahead in summer. If it is 09:00 in London in January, it is 18:00 in Seoul. In July, 09:00 London time (08:00 UTC) is 17:00 in Seoul.

Crossing midnight and the date

When the result goes below 0 or above 24, you have crossed a date boundary. The rule is easy:

If the result is negative, add 24 and move the date back by one day.
If the result is 24 or more, subtract 24 and move the date forward by one day.

Example: a meeting is set for 21:00 in Seoul on Tuesday. London in winter is 9 hours behind: 21 − 9 = 12:00 on Tuesday. New York is 14 hours behind Seoul in winter: 21 − 14 = 7:00 on Tuesday. Now try 03:00 Wednesday in Seoul: London is 03 − 9 = −6, so 18:00 on Tuesday, and New York is 03 − 14 = −11, so 13:00 on Tuesday. The same meeting is on different calendar days for people in different places. When you schedule an event, always write the date together with the time zone.

The date line and arriving "before you left"

The International Date Line is the place where the calendar date changes by one full day. Flying east across the Pacific you gain a day, and flying west you lose one. In practice you rarely need the date line itself; if you work through UTC, the dates sort themselves out automatically.

Example: a flight leaves Tokyo at 20:00 on Monday 12 January and takes 9 hours 40 minutes to reach Los Angeles. Tokyo 20:00 is 11:00 UTC. Add 9:40, and the plane lands at 20:40 UTC. Los Angeles is on UTC−8 in January, so the local time is 12:40 on Monday 12 January. By the wall clock, the plane arrived about 7 hours before it took off, still on the same Monday. Nothing impossible happened: you simply crossed the date line.

A flight duration example

Flight times printed on tickets are usually local times, so you cannot subtract departure from arrival directly. Convert both to UTC, or convert the arrival to the departure city's clock, and then subtract.

Example A: depart Seoul 10:00 on 15 January and land in New York at 10:00 on 15 January (local). That looks like zero hours of flight. Convert: Seoul 10:00 = 01:00 UTC. New York 10:00 on UTC−5 = 15:00 UTC. The flight lasted 15:00 − 01:00 = 14 hours.

Example B: the return, leaving New York at 13:00 on 15 January, with a flight time of 14 hours 40 minutes. New York 13:00 is 18:00 UTC. Add 14:40: 08:40 UTC on 16 January. Seoul is at UTC+9, so the local arrival time is 17:40 on 16 January. You left on the 15th and arrived on the 16th, even though the plane was in the air for less than a day.

A quick way to check: add the flight time to the departure time in the departure city's clock, then add the time difference. In example B, 13:00 + 14:40 = 27:40 New York time, which is 03:40 the next day in New York. Seoul is 14 hours ahead in winter, so add 14: 17:40 on 16 January. It matches.

Daylight saving time pitfalls

Daylight saving time (DST) moves clocks forward one hour in spring and back in autumn in some countries. Seoul, Tokyo and Mumbai do not use it. The big problem is that the countries that do use it switch on different dates.

The in-between weeks. In 2026, the United States moved its clocks forward on 8 March. The UK did not do so until 29 March. For those three weeks London and New York were only 4 hours apart instead of the usual 5. On 10 March at 14:00 in London (UTC+0), New York was at 10:00 (UTC−4).
The autumn gap. Similarly, the UK ended summer time on 25 October 2026 while the US ended it on 1 November, so London and New York were only 4 hours apart for that week too.
The same city, a different offset. The time gap between Seoul and London is 9 hours in winter and 8 hours in summer. Seoul and New York are 14 hours apart in winter and 13 in summer.
Southern Hemisphere. Countries such as Australia and New Zealand switch the other way round. Their summer is in December, so their offset changes at the opposite time of year.
The missing and repeated hour. On the spring switch day, one local hour does not exist (for example, 02:30 may be skipped), and on the autumn day one local hour occurs twice. A meeting during that hour needs an explicit UTC time.

The best protection is to store and compare times in UTC, and only convert to local time for display. If you must do it by hand, check whether each place uses summer time on the date in question, not just whether it does in general.

Adding and subtracting hours

Once times are in the same zone, normal clock arithmetic applies. The time calculator can add or subtract hours, minutes and seconds and take care of the carry over midnight. If you need to turn a duration such as 14 hours 40 minutes into a decimal for pay or billing, the time to decimal guide shows how: 40 minutes is 40 ÷ 60 ≈ 0.667 hours, so 14 h 40 min is about 14.67 hours. For more on adding and subtracting clock times, see the time calculation guide.

Common mistakes

Adding the offset instead of subtracting it. To go from local time to UTC, subtract the offset. For Seoul that means −9.
Ignoring the date. A correct time on the wrong day is still a missed meeting.
Assuming a fixed gap between two cities. It changes with DST, sometimes for just a few weeks.
Forgetting half-hour zones. India is UTC+5:30, so 09:00 UTC is 14:30 there.
Subtracting local times to get a flight duration. Convert both to the same zone first.
Using abbreviations like CST. It can mean Central Standard Time, China Standard Time or Cuba Standard Time. Prefer UTC offsets or city names.

FAQ

What is the difference between UTC and GMT?
For everyday use they are the same time. GMT is a time zone used in the UK in winter, while UTC is the international standard that scientists and computers use as the reference. London is on GMT in winter and on British Summer Time (UTC+1) in summer.

How do I know the offset of a city?
Search the city name together with "time zone", and check whether the result is for the date you care about, because DST changes the offset.

Why do some flights land "yesterday"?
Because the local clock at the destination can be behind the departure clock by more than the flight lasts. Flights from Asia to the Americas often arrive at an earlier clock time than they left.

What is the time difference between Seoul and New York?
14 hours in winter (Seoul is ahead) and 13 hours when New York is on summer time. For a quick check, use the time calculator.

Open the calculator: Time Calculator →