How to Convert Time Between Time Zones

Time zones look like simple arithmetic — add a few hours, subtract a few — and they generate an astonishing number of missed calls, meetings booked at 3 AM, and flights nearly missed. The trouble is that almost every intuitive assumption about them is wrong. This guide covers how time zones actually work, the four traps that cause nearly all the errors, and how to schedule across regions without embarrassing yourself.

Everything is an offset from UTC

There is one reference clock in the world: Coordinated Universal Time (UTC). Every time zone is defined as an offset from it. London in winter is UTC+0, New York is UTC−5, Dhaka is UTC+6, Tokyo is UTC+9.

This gives you a reliable method for any conversion: go through UTC. Convert the source time to UTC, then convert UTC to the destination. If it is 9:00 AM in New York (UTC−5), add 5 to get 2:00 PM UTC, then add 6 to get 8:00 PM in Dhaka.

Trying to jump directly between two zones by remembering “New York is 11 hours behind Dhaka” works right up until the moment it does not — which brings us to the first trap.

Trap 1: daylight saving destroys fixed maths

This is the big one. The gap between two cities is not constant.

Many regions shift their clocks forward an hour in summer and back in winter. Crucially, they do it on different dates, and a great many regions — including most of Asia and Africa — never do it at all.

So “New York is 5 hours behind London” is only true for part of the year. For a few weeks each spring and autumn, when one has shifted and the other has not, the gap is 4 hours. Meanwhile Dhaka never changes, so its offset from New York swings by an hour twice a year without Dhaka doing anything at all.

The practical rule: always convert for a specific date, never in the abstract. A conversion that was correct last month may be wrong today.

And note the southern hemisphere runs opposite: when the northern hemisphere springs forward, Australia is falling back. During the overlap the gap between London and Sydney changes by two hours.

Trap 2: the date rolls over

People convert the hours and forget the calendar. When zones are far apart, the day changes too.

A 9:00 AM Monday call in Los Angeles is 2:00 AM Tuesdayin Tokyo. Not Monday. If you send a colleague “Monday 9am my time” and they diligently convert only the hours, they will show up a full day late — or a full day early — and both of you will be certain you were right.

Always state and check the weekday and date alongside the time, in both zones. A good converter shows them; a mental calculation usually does not.

Trap 3: offsets are not whole hours

A quietly common assumption. Not every zone sits a round number of hours from UTC:

• India is UTC+5:30.
• Nepal is UTC+5:45 — a 45-minute offset.
• Iran is UTC+3:30, and parts of Australia are UTC+9:30.
• Chatham Islands in New Zealand are UTC+12:45.

And the full range is wider than people expect: from UTC−12 to UTC+14. That is a 26-hour spread — meaning at certain moments, three different calendar dates exist simultaneously on Earth.

Trap 4: abbreviations are ambiguous

Never schedule using letters alone. Time zone abbreviations are not unique:

• CST can mean US Central Standard Time (UTC−6), China Standard Time (UTC+8), or Cuba Standard Time. Three continents, one abbreviation.
• IST can mean India Standard Time (UTC+5:30), Irish Standard Time, or Israel Standard Time.
• EST vs EDT— people write “EST” year-round, but in summer New York is actually on EDT, an hour different. Half the “EST” meeting invites you receive in July are technically wrong.

The fix: name the city, not the abbreviation. “3pm New York time” is unambiguous and self-correcting for daylight saving. “3pm EST” is neither.

How to schedule a meeting across regions

1. Pick the exact date first, because daylight saving depends on it.
2. Choose an anchor zone— usually the organiser's — and state the time in it.
3. Convert to every participant's city, and check each one's weekday.
4. Look for the overlap. With a wide spread there may be no comfortable slot at all, and someone will have to take a call at an unpleasant hour. Better to know that in advance and rotate the pain than to spring it on people.
5. Send a calendar inviterather than a time in an email. A proper invite carries the timezone data and lands correctly in everyone's calendar automatically. This one habit eliminates most timezone mistakes outright.

A rough guide to overlaps: US East Coast and Europe share a workable window in the European afternoon. Europe and South Asia overlap in the European morning. US West Coast and Asia barely overlap at all — one side is always taking an early or a late call.

For developers: the golden rule

If you are storing times in software, there is one rule that prevents nearly every bug:

Store in UTC. Convert to local time only when displaying it.

A Unix timestamp has no timezone — it identifies an instant, and an instant is the same everywhere. Store local times in your database and your data becomes wrong twice a year, when an hour is either repeated or skipped and there is no way to tell which one you meant.

Never hard-code offsets either. Governments change timezone rules with little notice — countries have abolished daylight saving, changed their offset, or shifted their transition dates. Use the system timezone database, which is updated to track those changes.

Frequently asked questions

Is UTC the same as GMT? For everyday purposes, yes. Strictly, UTC is an atomic time standard and GMT is a time zone, but they are within a second of each other.

Why does the time difference between two cities change? Daylight saving. One city shifted and the other did not.

How many time zones are there? More than 24 — because of half-hour and 45-minute offsets, and the range extending to UTC+14, there are around 38 distinct offsets in use.

What is the safest way to state a time?Name a city and a date: “Tuesday 14 July, 3:00 PM New York time.” Or just send a calendar invite.

Convert time zones now

Use our Time Zone Converter to set a source time and compare it across as many cities as you like — daylight saving handled automatically for the exact date, with the weekday and GMT offset shown for each. Working with timestamps in code? The Epoch Converter handles Unix time, which is always UTC.