Why the time difference between two cities changes during the year
Most people carry one number in their head — “London is five hours ahead of New York.” For about 67% of major city pairs that number is wrong for part of every year.
The difference between two cities is only fixed if their clocks never move independently. As soon as one city starts or ends daylight saving time on a date the other doesn’t — or observes it at all when the other doesn’t — the gap between them shifts, usually by an hour, for a stretch of the year.
The four things that move the gap
Different dates. The United States and the European Union both change their clocks in spring and autumn, but not on the same days. For the two or three weeks in between, cities like London and New York sit an hour closer than their “usual” difference.
One side only. Much of the world doesn’t observe daylight saving at all — Tokyo, Phoenix, Dubai, most of India and Africa. Pair one of those with a city that does, and the gap simply steps back and forth twice a year as the other city’s clocks move.
Opposite hemispheres. When it’s summer in London it’s winter in Sydney. Their daylight-saving periods fall on opposite halves of the calendar, so the difference swings between two values for months at a time.
The odd ones out. A few places don’t play by the two-shifts- a-year rule — Morocco moves for Ramadan, Lord Howe Island shifts by thirty minutes rather than sixty, and countries occasionally abolish daylight saving outright, as Iran and most of Mexico did in 2022.
Four worked examples
London and New York
28 of 365 days are not the 5-hour difference most people assume.
London is 5 hours ahead of New York for most of the year. For 28 days — 8 Mar – 28 Mar; 25 Oct – 31 Oct — the difference narrows to 4 hours, because London and New York change their clocks on different dates.
| Dates | Difference |
|---|---|
| 1 Nov – 7 Mar | London 5 hours ahead |
| 8 Mar – 28 Mar | London 4 hours ahead |
| 29 Mar – 24 Oct | London 5 hours ahead |
| 25 Oct – 31 Oct | London 4 hours ahead |
London and Sydney
The difference between London and Sydney changes 4 times a year, ranging between 9 and 11 hours behind. London and Sydney observe daylight saving in opposite halves of the year.
| Dates | Difference |
|---|---|
| 25 Oct – 28 Mar | London 11 hours behind |
| 29 Mar – 4 Apr | London 10 hours behind |
| 5 Apr – 3 Oct | London 9 hours behind |
| 4 Oct – 24 Oct | London 10 hours behind |
New York and Phoenix
From 8 Mar to 31 Oct, New York is 3 hours ahead of Phoenix. For the rest of the year New York is 2 hours ahead of Phoenix, because Phoenix does not observe daylight saving time.
| Dates | Difference |
|---|---|
| 1 Nov – 7 Mar | New York 2 hours ahead |
| 8 Mar – 31 Oct | New York 3 hours ahead |
Berlin and Casablanca
Casablanca is the awkward one. Morocco stays on +01:00 all year but drops an hour for the month of Ramadan — a window set by the lunar calendar, so it moves about eleven days earlier each year. For those weeks the gap to Berlin closes, then re-opens.
| Dates | Difference |
|---|---|
| 25 Oct – 14 Feb | the same time |
| 15 Feb – 21 Mar | Berlin 1 hour ahead |
| 22 Mar – 28 Mar | the same time |
| 29 Mar – 24 Oct | Berlin 1 hour ahead |
How we keep this correct
These pages state thousands of facts about UTC offsets, and the underlying rules change by government decree. Every build is checked against the tz-edge-cases corpus — spring-forward gaps, fall-back ambiguity, sub-hour offsets, inverted daylight saving, the date line — and the build is refused if the time-zone data disagrees on any settled case. Nothing here is hand-typed; it is all recomputed from the IANA database on each deploy, so when a country changes its rules, the pages follow.