How time zones work, explained simply
The Earth rotates once every 24 hours, which means at any moment one side faces the sun and the other doesn't. To keep clocks roughly aligned with daylight, we divide the planet into time zones — strips of longitude that share the same official time.
Why time zones exist
Before standard time, every town set its clocks by local solar noon. That worked fine until trains and telegraphs made “what time is it where you are” an actually-useful question. The world adopted a system of zones anchored to Greenwich Mean Time (GMT) in the late 19th century, and we've been refining it ever since.
How zones are defined
Most zones are offsets from UTC (Coordinated Universal Time, GMT's modern successor) in whole hours: UTC−8, UTC+0, UTC+9. A few are offset by 30 or 45 minutes (India is UTC+5:30, Nepal is UTC+5:45). The boundaries follow national borders, not pure longitude — China spans five geographic zones but uses a single official time.
Daylight saving time
Many countries shift their clocks forward by an hour in spring and back in autumn to make better use of evening daylight. The shift dates vary by country, and many countries (most of Asia, most of Africa, all of Russia, Arizona in the US) don't observe DST at all. This is the source of about 90% of timezone confusion.
The IANA database
Behind every timezone-aware app sits the IANA Time Zone Database — a continuously-maintained list of every zone, every historical offset change, and every DST transition. When you see a name like Europe/London or America/New_York, that's an IANA identifier. Modern browsers ship it built in, which is how Zonelyy converts times accurately without needing to hardcode anything.
Common mistakes
- Confusing GMT with BST. London is on GMT in winter and BST (British Summer Time, +1) in summer. Saying “London is GMT” in July is wrong by an hour.
- Assuming “EST” means New York year-round. New York is on EST in winter and EDT in summer.
- Ignoring DST when scheduling recurring meetings. A 3pm-London meeting becomes a different US-time meeting twice a year if both regions don't shift on the same weekend.
The cleanest way to avoid all of this: pick a moment in UTC, then convert. That's what the converter does automatically.