Why Time Differences Change: Daylight Saving Explained
If London and New York are “5 hours apart,” why does a calendar invite sometimes land an hour off? The answer is daylight saving time — and once you see how it works, the moving gap makes sense.
What daylight saving time is
Daylight saving time (DST) is the practice of moving clocks forward by one hour in spring and back again in autumn, so that summer evenings have more daylight. When a region is “on” DST, its UTC offset increases by one hour: London shifts from UTC+0 (Greenwich Mean Time) to UTC+1 (British Summer Time); New York shifts from UTC−5 to UTC−4. The clocks change, but the reference clock — UTC — never does.
Who uses it, and when
DST is common across North America and Europe but far from universal:
- Europe (UK, EU): clocks go forward on the last Sunday of March and back on the last Sunday of October.
- United States & Canada (most): forward on the second Sunday of March, back on the first Sunday of November — different dates from Europe.
- Southern Hemisphere (e.g. Sydney): seasons are reversed, so clocks go forward around October and back around April.
- No DST at all: most of Asia, Africa and the Middle East. Dubai (UTC+4), Tokyo (UTC+9), Mumbai (UTC+5:30) and Singapore (UTC+8) keep the same offset all year.
Why the gap between two cities moves
A time difference only changes when the two cities are on different daylight-saving schedules. Three cases:
- Both shift on the same dates → the gap is steady. London and Paris are always 1 hour apart because they change together.
- One shifts, the other doesn’t → the gap swings by an hour across the year. London and Dubai are 4 hours apart in winter but 3 in summer, because only London changes. Watch it live on London → Dubai.
- Both shift, but on different dates → the gap is usually fixed, except for a short window each spring and autumn when one city has changed and the other hasn’t. London and New York are normally 5 hours apart but briefly 4. See London → New York.
How to stop meetings slipping by an hour
Recurring cross-border calls are where DST bites: a weekly meeting set “5 hours apart” quietly becomes 4 hours apart for a fortnight, and half the attendees show up at the wrong time. Two habits fix this:
- Anchor the meeting to one city’s local time, not to a fixed hour-gap. “10 AM London” stays correct through every clock change; “London time minus 5” does not.
- Check the live difference near the change dates (late March, late October, early November). Each city-pair page shows the current gap the moment it loads, so you never have to track which regions have flipped.
Related
- How to convert a time between any two cities — the method, DST included.
- UTC and GMT explained — the fixed reference behind every offset.
- Live examples: London → Dubai, London → Sydney, all world clocks.