How to Read a Time Zone Difference Correctly

Most missed remote meetings are not caused by anyone forgetting. They are caused by a conversion that was done correctly against the wrong assumption — an offset that was true in January and false in July, an abbreviation that means two different things, or a date that quietly moved.

This guide covers the assumptions that break, in the order they cause trouble.

UTC Is Not GMT, and London Is Not UTC

UTC — Coordinated Universal Time — is the reference against which every other zone is defined. It does not observe daylight saving and never changes. It is a standard, not a place.

GMT is a timezone that happens to equal UTC. The confusion arises because the United Kingdom uses GMT only in winter. From late March to late October, London is on British Summer Time at UTC+1. For seven months of the year, "GMT" written in an invite by someone in London is an hour off from what they meant.

If you are scheduling with London between April and October and someone writes "3 PM GMT", ask whether they mean 3 PM in London — which is 2 PM UTC — or 3 PM UTC, which is 4 PM in London. It is a genuine ambiguity, not pedantry.

The same problem affects every abbreviation. "CST" is US Central Standard Time, China Standard Time, and Cuba Standard Time. "IST" is India, Ireland and Israel. Abbreviations are unreliable identifiers; IANA zone names such as America/Chicago and Asia/Kolkata are not.

An Offset Is a Fact About a Moment, Not a Place

This is the single most useful idea in timezone handling. "New York is UTC−5" is not true — it is true in January and false in July, when New York is UTC−4. What is permanently true is that New York is in the America/New_York zone, which has rules determining its offset on any given date.

Once you internalise this, a lot of common errors become visible in advance:

• A wiki page listing team members' offsets is wrong for part of every year.
• A meeting described as "14:00 UTC, which is 9 AM for Sarah" stops being true when Sarah's region changes its clocks.
• A spreadsheet with a fixed offset column silently produces wrong answers twice a year.

Store the zone, compute the offset when you need it. This is exactly what we do — the methodology page explains how these pages recompute every figure rather than storing offsets.

Not Every Offset Is a Whole Number of Hours

A significant part of the world sits on a half-hour or quarter-hour boundary:

Any meeting scheduled on the hour elsewhere lands on the half hour — or the three-quarter hour — in these places. Since India alone represents an enormous share of globally distributed work, this is not an edge case.

Large Differences Are Date Problems

Once two cities are more than about eleven hours apart, they spend most of the day on different calendar dates. This turns a scheduling question into a genuinely ambiguous one.

Consider Los Angeles and Sydney. A meeting at 4:00 PM Monday in Los Angeles is 9:00 AM Tuesday in Sydney. If someone says "let's do Tuesday morning", the two parties may be discussing moments 24 hours apart, and both will believe they have agreed.

The Southern Hemisphere Runs the Other Way

Daylight saving is not a global event. Australia, New Zealand, Chile and parts of southern Africa and South America shift their clocks in the opposite season to Europe and North America. Sydney moves forward in October, when London is moving back.

The consequence is that the gap between a northern and a southern city changes by two hours across the year, not one. London and Sydney are nine hours apart in the northern summer and eleven in the northern winter. A meeting that worked comfortably in July can be impossible in January.

Our southern hemisphere DST guide covers this in detail, including the weeks when both hemispheres are on summer time simultaneously.

Many Places Do Not Observe DST at All

It is easy to assume clock changes are universal. They are not — most of Asia and Africa have never observed daylight saving or abandoned it decades ago, and several countries have dropped it recently:

• Russia abolished seasonal clock changes in 2014.
• Turkey abolished them in 2016.
• Brazil abolished them in 2019.
• Mexico abolished them for most of the country in 2022.
• Japan, China, India, Singapore and the Gulf states do not observe them and have not for decades.

When one side of a pair observes DST and the other does not, the relationship between them changes twice a year even though only one clock moved. This is the most common source of surprise on corridors people assume are stable — a São Paulo–London meeting shifts because London moved, not Brazil.

Arithmetic Across Midnight

Manual conversion goes wrong most often when it crosses midnight, because the date has to change and mental arithmetic tends not to notice. The reliable method is to convert through UTC in two steps rather than computing the difference directly:

1. Convert the source time to UTC by subtracting its offset.
2. Convert UTC to the destination by adding its offset.
3. If the result is below zero, add 24 and move back a day. If it is 24 or above, subtract 24 and move forward a day.

Worked example — 9:00 PM Tuesday in Tokyo (UTC+9), converting to Los Angeles (UTC−7 in summer):

• 21:00 − 9 = 12:00 UTC, still Tuesday.
• 12:00 − 7 = 05:00 in Los Angeles, still Tuesday.

And the reverse — 9:00 AM Tuesday in Los Angeles to Tokyo:

• 09:00 + 7 = 16:00 UTC Tuesday.
• 16:00 + 9 = 25:00, which is above 24, so 01:00 Wednesday in Tokyo.

The Five Mistakes Worth Memorising

1. Using an abbreviation instead of a city or zone. CST and IST each mean at least three different things.
2. Treating an offset as permanent. It is a property of a moment, not a place.
3. Assuming GMT means London time. True in winter only.
4. Dropping the half hour. India, Nepal, central Australia and Newfoundland are not on hour boundaries.
5. Agreeing a day without agreeing whose day. Beyond eleven hours apart, "Tuesday" is ambiguous.

Related reading

• DST changes 2026 — the mismatch weeks and exact dates
• Southern hemisphere DST — why the gap changes by two hours
• Our methodology — how these figures are computed
• All time difference pages — worked overlap for specific city pairs