How Daylight Saving Time Affects Global Meetings
Daylight saving time, also called summer time, moves clocks forward by an hour in spring and back in autumn. Inside one country it is a minor nuisance. Across countries it quietly changes the gaps between cities, because different regions switch on different dates and many regions never switch at all. This guide explains how that works and how to keep global meetings on track.
Who changes clocks, and when
The United States and most of Canada move clocks forward on the second Sunday in March and back on the first Sunday in November, at 2:00 AM local time.
The United Kingdom and the European Union move clocks forward on the last Sunday in March and back on the last Sunday in October, at 01:00 UTC, so every country in the bloc changes at the same moment.
In the southern hemisphere the seasons are reversed. Sydney and much of south-east Australia move forward on the first Sunday in October and back on the first Sunday in April. Queensland and the Northern Territory do not change.
Many major business hubs never change their clocks: India, the UAE, Saudi Arabia, Singapore, Hong Kong, China, Japan and South Africa all keep the same time all year. Brazil ended daylight saving time in 2019, so São Paulo now stays on UTC−3 year-round.
Why the gap between two cities changes
If both cities switch on the same dates, their gap stays the same. If only one switches, the gap changes by an hour twice a year. London is 4 hours 30 minutes behind Mumbai in summer but 5 hours 30 minutes behind in winter, because only London changes.
When both switch on different dates, there are short "shoulder" periods where the usual gap is wrong. New York and London are normally five hours apart, but for two or three weeks in March, after the US has moved forward and before Europe does, the gap is four hours. The same happens for about a week around the end of October and start of November.
Between the hemispheres the effect is larger. Sydney and London can be 9, 10 or 11 hours apart depending on the time of year, because one is moving forward while the other moves back.
What this does to recurring meetings
A calendar invite is anchored to the organiser's time zone. If the organiser is in New York and books a weekly 10:00 AM call, it stays at 10:00 AM in New York all year, but moves by an hour for colleagues in London during the shoulder weeks and for colleagues in Mumbai at each US change. Attendees who copied the time into their diary by hand, or who rely on a fixed mental offset, are the ones who miss the call.
How to protect your schedule
Create every meeting in a calendar that uses a real, named time zone rather than a fixed offset such as UTC−5. Named zones like America/New_York or Europe/London carry the full set of rules and move correctly.
Mark the changeover dates in your team calendar: the second Sunday in March, the last Sunday in March, the last Sunday in October and the first Sunday in November, plus the October and April dates if you work with Australia. In the week before each date, check recurring meetings with international attendees.
Decide which location the meeting is anchored to. For a meeting that must suit a client in London, anchor it to London time so the client's slot never moves, and let the organiser's local time adjust instead.
When in doubt, check a converter for the specific meeting date rather than today's date. Tools that use the IANA time zone database, as WorldTimeFX does, calculate the offset for the exact moment you choose.
Rules can change
Governments change their time rules more often than people expect. Countries regularly debate ending clock changes, and when they do, software needs an update to the time zone database. Keeping browsers and operating systems current ensures those rule changes are picked up, which is one reason a live converter is safer than a printed offset table.