The Four Weeks a Year When London and New York Are Not Five Hours Apart
Most people who work across the Atlantic carry a number in their head. London is five hours ahead of New York. It is correct for about 48 weeks of the year, and quietly wrong for the other four, because the United States and Europe do not change their clocks on the same day.
The mismatch, precisely
The two regions use different rules, and have done since the US moved its dates in 2007.
- United States: clocks go forward on the second Sunday in March and back on the first Sunday in November.
- European Union and United Kingdom: clocks go forward on the last Sunday in March and back on the last Sunday in October.
In spring, the US jumps first and Europe follows up to three weeks later. In autumn, Europe falls back first and the US follows about a week later. In both gaps the usual difference shrinks by an hour.
Take 2026. The US moves on 8 March, the EU and UK on 29 March. For those three weeks New York is on daylight time at UTC-4 while London is still on GMT at UTC+0, so the gap is four hours rather than five. In autumn, Europe falls back on 25 October while the US waits until 1 November, so that week is four hours as well.
Four weeks a year, give or take, the number in your head is wrong. And it is wrong in the direction that makes you an hour late rather than an hour early.
The southern hemisphere makes it worse
Australia and New Zealand run daylight saving in their own summer, which is the northern winter. Their clocks move in the opposite direction to Europe's, at yet another set of dates.
The result is that the London to Sydney difference is not a number at all, it is a range. Across the year it moves between 9 and 11 hours, and it passes through every value in between depending on which hemisphere has switched and which has not. Anyone who has tried to hold a standing weekly call between Europe and Australia has discovered this the hard way, usually in October.
The Auckland and London comparison recomputes the gap live rather than quoting an average, which is the only way to get it right in the weeks around a transition.
Most of the world does not do this at all
It is worth remembering that daylight saving is a minority practice. Roughly a third of countries observe it, concentrated in Europe, North America and parts of Oceania and South America. Most of Asia and nearly all of Africa do not.
This means a transition does not shift your relationship with the whole world evenly. When European clocks move in March, the gap to New York closes by an hour, the gap to Tokyo opens by an hour, and the gap to Lagos opens by an hour. A single change ripples differently in every direction, which is precisely why trying to hold the whole picture in your head does not work.
The hour that does not exist, and the hour that happens twice
There is a second, sharper problem on the transition day itself.
When clocks go forward, an hour is deleted. In the US spring transition the local time jumps from 01:59:59 to 03:00:00, so 02:30 does not occur. A meeting scheduled for that instant has no valid time to happen at, and different calendar systems resolve it differently: some fire an hour early, some an hour late, some silently drop it.
When clocks go back, an hour repeats. Local time reaches 01:59:59, returns to 01:00:00 and runs through the same hour again. A timestamp of 01:30 that day is genuinely ambiguous: it describes two different moments an hour apart. This is a recurring source of bugs in logging, billing and anything that assumes timestamps increase.
How to not get caught
Three habits remove nearly all of the risk.
- Schedule in a named zone, never in an offset. Write "09:00 Europe/London", not "09:00 UTC+1". The offset is a fact about a moment, not about a place, and it changes underneath you. The named zone survives the transition.
- Treat the transition weeks as suspect. If a call falls in mid-March, late March, late October or early November, check the actual difference rather than trusting the usual one.
- Confirm the other side's local time, not yours. Most transition failures are one party applying their own country's change to someone else's clock.
The London and New York comparison calculates the gap from the offsets actually in effect at the moment you load it, so during those four weeks it shows four hours rather than the remembered five. It also works out which hours of the working day genuinely overlap, which is the question most people are really asking.