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Guide

Daylight saving time, explained properly

Daylight saving is the single biggest source of scheduling errors between countries. Not because the rule is complicated, but because only part of the world follows it, and those that do change on different dates. This guide sets out who changes, when, and what it does to a recurring meeting.

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Who changes, and when

Roughly 70 countries observe some form of seasonal clock change, and most of the world's population does not. The United States and Canada shift on the second Sunday in March and the first Sunday in November. The European Union and the United Kingdom shift on the last Sunday in March and the last Sunday in October. Those dates do not line up, which creates two windows each year — a few weeks in spring and one in autumn — when the gap between North America and Europe is an hour different from usual.

The southern hemisphere runs the opposite way. Australia's south-eastern states and New Zealand move forward in October and back in April, so while Europe is gaining an hour, Sydney is losing one. For part of the year the gap between London and Sydney is nine hours; for another part it is eleven.

Most of Asia, Africa and South America does not change clocks at all. India, China, Japan, Pakistan, the Gulf states, Brazil and Russia all keep a single offset year-round. This is why a London–Mumbai meeting drifts twice a year while a Mumbai–Dubai meeting never does: the drift comes from one side changing while the other stands still.

Key facts

It is a shift, not a new zone

The clock moves, the zone identifier does not — America/New_York covers both EST and EDT.

The abbreviation changes

EST becomes EDT, GMT becomes BST, CET becomes CEST. Same place, different label.

Dates differ by region

North America changes in March and November; Europe in late March and late October.

The southern hemisphere is inverted

Australia and New Zealand gain an hour in October, not March.

Most of the world opts out

India, China, Japan, most of Africa and most of South America never change.

Two awkward windows

For a few weeks each year the usual gap between Europe and North America is off by one hour.

How to get it right

1

Identify both zones properly

Use the IANA identifier such as Europe/London, not the abbreviation.

2

Check whether each observes DST

If only one does, the gap between them is seasonal rather than fixed.

3

Note the changeover dates

The two regions rarely change on the same weekend.

4

Send calendar invitations

Store the instant, not the clock time, and every attendee sees the right slot automatically.

Questions people ask

They describe the same region in different seasons. EST is UTC−5 and applies from November to March; EDT is UTC−4 and applies from March to November. Most people who write EST actually mean Eastern Time in general.

Keep reading

UTC, GMT and Zulu time

What UTC actually is, how it differs from GMT, why aviation says Zulu, and when the distinction matters.

Scheduling across time zones

A practical method for booking meetings across zones: finding the overlap, rotating the pain, and stopping recurring calls from drifting.

Why abbreviations are ambiguous

IST means three different things, CST means three more. Which abbreviations are ambiguous and what to use instead.

Check a specific conversion

Every pair page shows both the standard-time and daylight-saving answer.

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