Daylight saving time, explained

Daylight saving time (DST) is the practice of moving clocks forward by an hour in spring and back by an hour in autumn, so that evenings get an extra hour of daylight during the warmer months at the cost of darker mornings. It's the single biggest source of confusion in timezone conversion, because the "gap" between two cities isn't fixed — it can change twice a year, and not always on the same date for both cities.

Why it exists

The modern version was popularized during the First World War as a way to conserve fuel by better matching waking hours to daylight hours. It's stuck around in many regions since, though the actual energy savings are debated, and a growing number of places have dropped it entirely.

Who observes it — and who doesn't

Most of the United States, Canada, and Europe observe DST, but on different schedules: the EU shifts on the last Sunday of March and October, while the US and Canada shift on the second Sunday of March and the first Sunday of November. Most of Asia, most of Africa, and all of the countries near the equator skip DST entirely, since day length there barely changes across the year anyway. Australia observes it, but only in the states that choose to (Queensland, Western Australia, and the Northern Territory opt out) — and because Australia is in the southern hemisphere, its DST months are the reverse of the northern hemisphere's.

Why this breaks conversions people do "in their head"

If you remember that New York is "5 hours behind London," that's only true for part of the year. The US shifts to DST about two to three weeks before the UK does in spring, and shifts back about one week before the UK does in autumn — so for those short windows, the gap is 4 hours instead of 5. Multiply that kind of mismatch across dozens of country pairs, each with their own DST start/end dates, and it's easy to see why a fixed mental offset eventually gets something wrong — usually a meeting invite that's an hour off for a week or two around the transition.

How to avoid it

The reliable fix is to convert using the actual calendar date of the event, not a remembered offset. Tools (including the converter on this site) that use the IANA timezone database apply the correct historical or scheduled DST rule for that specific date automatically, which is the same approach your phone and calendar apps use internally.