Published 1 January 2025 · 6 min read

How to Convert Time Zones — A Simple Guide for Global Teams

You're trying to schedule a call with colleagues in New York, Sydney, and London. You pick 2pm your time, fire off the invite, and feel quite pleased with yourself. Then the replies start coming in: "That's 4am for me." "I think that's actually 3am — isn't it DST over there?" Before you know it, there's a 12-message thread and nobody's sure what time the meeting actually is. We've all been there, and it's properly annoying.

What's UTC and why should you care?

UTC (Coordinated Universal Time) is the baseline that every time zone is measured against. Think of it as the "zero point" on the time zone number line. London sits at UTC+0 in winter (that's GMT), then shifts to UTC+1 when British Summer Time kicks in. New York is UTC-5 in winter (EST) and UTC-4 in summer (EDT).

To convert between two cities, you just work out the gap between their UTC offsets. If London's at UTC+1 and New York's at UTC-4, that's a 5-hour difference — so 3pm in London is 10am in New York during summer. Simple enough when the numbers don't change. The problem is, they do change.

The daylight saving trap (this is where it gets messy)

Here's the thing most people get wrong: they assume the time difference between two cities stays the same all year. It doesn't. And it's not just a matter of remembering "spring forward, fall back" — the UK and the US change their clocks on different weekends.

• UK and Europe — clocks change on the last Sunday in March and the last Sunday in October.
• US and Canada — clocks change on the second Sunday in March and the first Sunday in November.
• Australia — clocks change in April and October (they're in the southern hemisphere, so their seasons are flipped).
• Japan, India, China — don't observe DST at all. Lucky them.

This creates a genuinely confusing situation. The London-to-New York gap is usually 5 hours, but for a few weeks in spring and autumn — when one country has changed its clocks and the other hasn't yet — it drops to 4 hours. If you've got a standing weekly meeting, it'll randomly shift by an hour twice a year, and nobody will warn you.

A practical example

Let's say you've got a team meeting every Thursday at 3pm London time, and your colleague in New York usually joins at 10am their time. In late March, the UK springs forward but the US hasn't yet. Suddenly that 3pm London meeting is only 4 hours ahead of New York instead of 5 — so your colleague needs to join at 11am, not 10am. Two weeks later, the US springs forward too, and it goes back to normal. Miss this, and someone's either an hour late or sitting in an empty meeting room. It's happened to all of us.

The half-hour zones nobody tells you about

Most people think time zones move in neat one-hour steps. They don't. India is UTC+5:30, which means when it's noon in London (winter), it's 5:30pm in Mumbai — not 5pm or 6pm, but half past. Nepal goes one further at UTC+5:45. Parts of Australia use UTC+9:30. The Chatham Islands in New Zealand are at UTC+12:45, which is frankly just showing off.

These half-hour (and quarter-hour) offsets catch people out all the time, especially when you're mentally calculating meeting times. You think "India's about 5 hours ahead" and schedule accordingly, only to realise your meeting invite is 30 minutes off.

Common mistakes to avoid

• Using a fixed offset year-round — always check the offset for the specific date you're converting. "London is 5 hours ahead of New York" is only true for about 9 months of the year.
• Forgetting the date might change — if it's 11pm on Tuesday in London, it might already be Wednesday morning in Tokyo, or still Tuesday afternoon in Los Angeles. This trips people up more often than you'd think.
• Not double-checking around DST transitions — the two weeks after clock changes are prime time for scheduling disasters. Put a reminder in your calendar.
• Ignoring half-hour zones — India alone has over a billion people on a half-hour offset. Don't round it.
• Saying "3pm" without specifying the zone — if your team spans more than one time zone, always write "3pm GMT" or "3pm EST." Just "3pm" is useless.

How time zones actually work (a quick history)

Before time zones were a thing, every town set its own clock based on when the sun was highest. That was fine when the fastest you could travel was by horse, but railways absolutely broke the system. Train timetables were chaos when every station along the route used a slightly different "local noon."

In 1884, representatives from 25 countries got together in Washington and agreed to split the world into 24 standard zones, each covering 15 degrees of longitude, with Greenwich in London as the zero point. It took decades for everyone to actually adopt it, but here we are.

The reason we've got more than 24 zones today is that countries like India and Nepal chose offsets that better matched their actual solar noon rather than rounding to the nearest hour. It's more accurate for them, even if it makes everyone else's maths harder.

Some genuinely odd time zone facts

China covers five geographical time zones but uses just one — UTC+8 (Beijing Time) — across the entire country. That means in western China, the sun might not come up until mid-morning by the clock. Imagine your alarm going off at 7am and it's still pitch dark outside.

Kiribati, a small island nation in the Pacific, sits on the International Date Line and chose to put all its territory on the same side. The result? They've got a time zone of UTC+14, making them the first place on Earth to see each new day. On New Year's Eve, Kiribati celebrates a full 26 hours before Hawaii does.

And here's a fun one for your next meeting: during DST transitions, when clocks fall back, the same local time happens twice. If you schedule something for 1:30am on the night the clocks go back, which 1:30am do you mean? The first one or the second one? Nobody knows, which is why we should all just avoid scheduling anything during clock changes.

Tips for getting it right every time

After years of working across time zones, here's what actually works:

• Always include the time zone in your invite. "Let's meet at 3pm GMT" is clear. "Let's meet at 3pm" is an invitation for confusion.
• Use UTC as a common reference. When you've got people in five countries, converting everything to UTC first and then to each local time saves headaches.
• Set your calendar time zone properly. Google Calendar and Outlook both auto-convert times for recipients, but only if everyone's time zone is set correctly. Worth checking yours right now, honestly.
• Flag DST transitions. Add a calendar reminder for the weekends when clocks change in your team's countries. You'll thank yourself.
• Record important meetings. If your team spans London, New York, and Tokyo, there's basically no time that's civilised for everyone. Record the call and let people in awkward time zones watch it at a reasonable hour instead.
• Use a proper converter. Mental maths is great until you forget about that half-hour offset or that DST transition you didn't know about. A tool that handles it all automatically is worth its weight in gold.

For teams spread across the globe, we'd also recommend finding your "overlap window" — those precious couple of hours when most of the team is awake and working. Protect that window fiercely for live collaboration, and use async methods (recorded updates, shared docs, messaging) for everything else. It's the only way to stay sane when you've got a 12-hour spread across the team.

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