MeetingTimePro

Why the time difference between cities changes

New York is five hours behind London — except when it is four. Drag through the year below and watch it happen.

Place A: New York

Place B: London

Tue, March 17, 2026

New YorkEDT UTC−4
LondonGMT UTC+0
London is4 h ahead

Not the usual gap (5 h ahead). One place has changed its clocks and the other has not yet — a recurring meeting booked in one zone moves by an hour for the other.

Jump to a change:

Three rules explain nearly every surprise

1. Not everyone changes clocks. India, China, Japan and most of Africa never shift. So the gap between Bengaluru and New York is 9½ hours in summer and 10½ in winter: only New York moved.

2. The ones who do, change on different weekends. The US switches on the second Sunday in March, the EU on the last Sunday. For those weeks the usual 5-hour New York–London gap becomes 4. The same thing happens in reverse between the last Sunday in October and the first Sunday in November.

3. The Southern Hemisphere runs the other way. Sydney springs forward in October, just as London falls back. The Sydney–London gap slides from 9 hours to 11 across a few weeks.

What this means for meetings

  • A recurring meeting follows the organiser’s clock. Attendees elsewhere see it shift by an hour on change weekends. Warn people a week ahead.
  • Always check a proposed time on the actual date. Every tool on this site does that: the meeting planner, the time zone converter and the time difference calculator read the IANA database for the day you pick.
  • Say “9:00 New York time”, not “9:00 EST”. In July New York is on EDT, and many people who write EST mean whatever the New York clock says.

Where the rules come from

Clock rules are set by governments and change more often than you might expect; the IANA time zone database (often called the tz or Olson database) tracks every change since 1970 and ships inside every major operating system and browser. That is why this site does not store any offsets of its own. It asks your browser.

Governments can move a clock by decree; they cannot move the sky. Why the Moon wears a different shape each night is one of the explainers at ahaboo, narrated and interactive, with no politics involved.

Questions people ask

When do the US and Europe change their clocks?

The United States and Canada spring forward on the second Sunday of March and fall back on the first Sunday of November. The European Union and the UK switch on the last Sunday of March and the last Sunday of October, at 01:00 UTC. That leaves roughly three weeks in March and one week around the start of November when the usual transatlantic gap is an hour shorter.

Which countries do not use daylight saving time?

Most of Asia and Africa, including India, China, Japan, Singapore and South Africa, keep the same clock all year. So do Iceland, most of Arizona in the US, and the Australian state of Queensland.

Why does a recurring meeting move by an hour?

Calendar apps anchor a recurring meeting to the organiser’s time zone. When the organiser’s clocks change but an attendee’s do not (or change on a different date), the meeting lands an hour earlier or later on the attendee’s clock.

Why does the Southern Hemisphere change clocks in the opposite months?

Places far south of the equator have their long summer days from October to March, so Sydney, Melbourne, Auckland and Santiago move clocks forward around September–October and back around April. The gap between Sydney and London can swing by two hours across the year.