Why does London say UTC+1 in summer?

The UK uses British Summer Time from the last Sunday of March to the last Sunday of October, which pushes the clock one hour ahead of Greenwich Mean Time. So while London's standard offset is UTC+0, its actual offset for about half the year is UTC+1.

What is the difference between a UTC offset and daylight saving time?

An offset is the fixed count of hours east or west of UTC that defines a zone's standard time. Daylight saving is a seasonal rule that temporarily shifts some zones' clocks — usually +1 — so the offset changes twice a year. Confusing the two is the most common time zone mistake.

Do all time zones use daylight saving time?

No. Some, like Shanghai, Tokyo and most of India, keep a constant offset all year. Only regions that adopt DST change their offset twice a year, and they do not always do it on the same dates — which is exactly when conversions fail.

UTC Offset vs DST: Why London Is UTC+1 in Summer

Here is a tiny quiz. London is at UTC+0, right? Half of the year — yes. The other half, the whole country quietly runs an hour early at UTC+1. Travelers who memorized "GMT is zero" and never looked again build schedules that drift by sixty minutes without understanding why.

The problem is that two different concepts get fused into one phrase. Nail the split and every conversion becomes easier to sanity-check by hand.

The offset is fixed, boring, and reliable

A UTC offset is simply "how many hours east or west of UTC is this zone's standard time." Paris sits at UTC+1, Tokyo at UTC+9, New York at UTC−5. This number is stable and defines the zone on a map. Think of it as the address of a place in time coordinates.

DST is a seasonal override

Daylight saving time is a rule some regions attach on top of the offset. It pushes the clock one hour forward for part of the year and back for the other part. Britain calls the summer rule British Summer Time, which is why London reads UTC+1 rather than its standard UTC+0 from late March to late October.

Traps appear because regions do not all flip on the same weekend. Even within one country, not every state participates. When you rely on a memorized offset during a crossover month, you are one weekend out of date.

Two tables that look alike, meaning different things

City	Standard offset	Seasonal change
London	UTC+0	+1 (March–Oct)
Paris	UTC+1	+1 (March–Oct)
Shanghai	UTC+8	None
New York	UTC−5	+1 (Mar–Nov)

Notice the pattern: a whole region rotates together. London and Paris both add one hour the same weekend, so their difference stays at one hour all year. The real murderers of schedules are the late movers — see why DST breaks meetings for weeks.

Because offsets and DST rules are stateful, the reliable habit is to always convert at the moment you schedule, never from memory. Add every participant's place to the HourSwift converter and the current offset is read live, with each DST-active zone tagged. Related: how to publish one time a global audience reads correctly.