Time zones and daylight saving time: meetings across cities

Updated: 28 October 2026

The first part of this series explained UTC, offsets and the basics of daylight saving time; the second, how to count days when a day can have 23 or 25 hours. This part is about the practical side: why the gap between two cities changes during the year, which countries move their clocks and when, which local times never happen or happen twice, and how to write, store and schedule a time so that it lands at the right moment everywhere. The 2026 dates below were computed from the IANA time zone database, release 2026e.

Offset, time zone and abbreviation

Three different things are often called a time zone. An offset is a number: UTC+1 means one hour ahead of UTC. A time zone is a place with its full set of rules: Europe/Lisbon is UTC+0 in winter and UTC+1 in summer, and it also records every past change. An abbreviation such as CET or EST is a short label for one of those offsets, used in conversation.

Abbreviations are the least reliable of the three. The IANA time zone database itself warns that CST means one thing in China and another in North America, and that IST can mean India, Ireland or Israel. Its zone names, written as a region and a city — Europe/Lisbon, America/Sao_Paulo, Asia/Tokyo — avoid that, and they use cities rather than countries so that a name stays correct when borders or rules change.

Abbreviation	Can mean	Offset
`CST`	Central Standard Time (Chicago, Mexico City)	UTC−6
`CST`	China Standard Time (Shanghai, Beijing)	UTC+8
`CST`	Cuba Standard Time (Havana)	UTC−5
`IST`	India Standard Time	UTC+5:30
`IST`	Irish Standard Time (Ireland in summer)	UTC+1
`IST`	Israel Standard Time	UTC+2

In software, store the zone name. In a message to people, write the city and the offset: “15:00 Lisbon time (UTC+1)” is read correctly anywhere; “15:00 IST” is not.

One country, several clocks

A country is not a time zone. Portugal has two: mainland Portugal and Madeira keep the same time, while the Azores are one hour behind all year; both change their clocks on the same days, as the Portuguese meteorological institute IPMA explains under Decree-Law 17/96. Spain’s Canary Islands are likewise one hour behind mainland Spain.

Brazil has four zones, set by a 1913 decree last amended in 2013: Fernando de Noronha at UTC−2; the Federal District, with Brasília, and most of the country at UTC−3; Mato Grosso, Mato Grosso do Sul, Rondônia, Roraima and eastern Amazonas at UTC−4; Acre and western Amazonas at UTC−5. None of them has changed its clocks since 2019. The United States, Canada, Mexico, Russia and Australia also span several zones.

Time zone	Where	January	July
`Europe/Lisbon`	Mainland Portugal and Madeira	UTC+0	UTC+1
`Atlantic/Azores`	Azores	UTC−1	UTC+0
`Europe/Madrid`	Mainland Spain	UTC+1	UTC+2
`Atlantic/Canary`	Canary Islands	UTC+0	UTC+1
`America/Noronha`	Fernando de Noronha	UTC−2	UTC−2
`America/Sao_Paulo`	Brasília, São Paulo, Rio de Janeiro	UTC−3	UTC−3
`America/Manaus`	Manaus	UTC−4	UTC−4
`America/Rio_Branco`	Acre	UTC−5	UTC−5

Who changes the clocks, and when

Each country decides whether and when its clocks change, so the dates differ. In 2026:

Where	Clocks forward	Clocks back	Rule
European Union, including Portugal, Spain, France, Germany and Italy	29 March	25 October	Last Sunday of March and of October, at 01:00 UTC everywhere at once
United Kingdom	29 March	25 October	The same days and time as the EU
United States, except Hawaii and most of Arizona	8 March	1 November	Second Sunday of March and first Sunday of November, at 2:00 local time
New South Wales (Sydney), Australia	4 October	5 April	First Sunday of October and first Sunday of April, at 2:00 local time
Brazil	—	—	No daylight saving time since 2019
Japan, China and India	—	—	No daylight saving time

Because the EU changes at the same instant everywhere, the clocks jump at 01:00 in Lisbon and London but at 02:00 in Paris, Berlin, Madrid and Rome. In the United States each zone changes at 2:00 its own time, so the change crosses the country over several hours. In the southern hemisphere summer falls at the end of the year, so Sydney moves its clocks forward in October and back in April.

The rules themselves can change. In 2018 the European Commission proposed ending the seasonal clock changes, and in 2019 the European Parliament backed the proposal, but the Council of the EU has not yet agreed its position, so the twice-yearly change continues.

The weeks when the gap changes

When two places change their clocks on different dates, or only one of them changes, the difference between them moves during the year. In 2026:

Between	Difference	When in 2026
Lisbon and New York	5 hours	Most of the year
Lisbon and New York	4 hours	8–29 March and 25 October – 1 November
Lisbon and São Paulo	3 hours	Until 29 March and from 25 October
Lisbon and São Paulo	4 hours	29 March – 25 October
Lisbon and Sydney	11 hours	Until 29 March and from 25 October
Lisbon and Sydney	10 hours	29 March – 5 April and 4–25 October
Lisbon and Sydney	9 hours	5 April – 4 October

So a weekly call at 15:00 in Lisbon is at 10:00 in New York most of the year, but at 11:00 during those weeks. With working hours of 9:00 to 17:00 on both sides, the shared window is three hours (14:00–17:00 in Lisbon) most of the year and four hours in those weeks. Paris, Berlin, Madrid and Rome have the same weeks with New York, with a six-hour gap that becomes five.

Times that never happen, and times that happen twice

On the night the clocks go forward, an hour of local time is skipped; on the night they go back, an hour is repeated. In 2026:

Lisbon and London: 01:00–01:59 does not exist on 29 March and happens twice on 25 October.
Paris, Berlin, Madrid and Rome: the same for 02:00–02:59, on the same two days.
New York: 02:00–02:59 does not exist on 8 March, and 01:00–01:59 happens twice on 1 November.

Software has to decide what such a time means. The iCalendar standard used by calendar invitations, RFC 5545, sets a rule: a time that does not exist is read with the offset in force before the gap, so 01:30 in Lisbon on 29 March is taken as 01:30 UTC, which the clock shows as 02:30; a time that happens twice means its first occurrence. Other programs may choose differently.

For alarms, backups and other scheduled jobs, avoid those hours, or schedule the job in UTC, which has no gaps or repeats.

Storing and sending a future time

For something that has already happened — a payment, a log entry — store the instant in UTC or as a timestamp, as the first part explains. For something planned in a place — a meeting at 10:00 in Lisbon next January — store the local time and the zone name, and convert only when you need to show or compare it. If you store only the UTC instant and the place changes its rules before the date, the meeting moves.

Rules do change, often at short notice. In 2026 alone, British Columbia, Alberta, Manitoba and the Northwest Territories in Canada stopped changing their clocks, and Morocco went back to UTC all year from 20 September; the IANA time zone database published five releases between March and September to follow changes like these. Phones, browsers and servers learn about a change only when their time zone data is updated, so an older device may still show the old time for those places.

2027-01-15T10:00:00+00:00[Europe/Lisbon]RFC 9557 (2024): the local time, its offset and, in brackets, the zone it belongs to.
2027-01-15T10:00:00ZUTC only: right for a past instant, fragile for a future meeting.
15 January 2027, 10:00 Lisbon time (UTC+0)For people: a city and the offset, never an abbreviation alone.

The bracketed form lets a program that knows the zone check that the offset still matches the current rules before it trusts the time.

Scheduling a meeting across time zones

A few habits prevent most mistakes:

Create the meeting in a calendar, not in the text of an email: an invitation stores the time zone, and each guest’s calendar converts it.
Set a recurring meeting in the organiser’s time zone, and warn the other side that it moves by an hour for a few weeks in spring and autumn.
Write times as a city and an offset — 15:00 Lisbon (UTC+1) — and avoid abbreviations.
Check the overlap of working hours close to the actual date, not on the day you plan it.
For dates far ahead, keep devices up to date: a rule change reaches them only through a time zone data update.

The Meeting time overlap tool takes two or three IANA zones and the working hours, applies each zone’s daylight saving time for the current date, and lists the hours that suit everyone, in UTC and in each local time.

Meeting time overlap

Frequently asked questions

Is GMT the same as the time in London?

Only in winter. From the last Sunday of March to the last Sunday of October the United Kingdom uses British Summer Time, UTC+1. GMT itself never changes, which is why it works as a reference.

Why did my meeting move by an hour?

Because the two places changed their clocks on different dates, or only one of them changes. Between Europe and the United States this happens for about three weeks in March and one week around the end of October.

Should I use UTC+1 or Europe/Paris?

Europe/Paris for anything in the future or anything that repeats: the zone follows the summer and winter changes, a fixed offset does not. A fixed offset is right only for a single instant that is already settled, such as a timestamp.

Does Brazil still have daylight saving time?

No. It was abolished by Decree 9.772 of April 2019, and Brazil’s four time zones keep the same offset all year.

Why do two devices show a different time for the same city?

One of them probably has older time zone data. Places that changed their rules recently, such as parts of Canada and Morocco in 2026, are shown correctly only after the update.

Previous in the series ← Counting dates: days between dates, leap years and ISO weeks