Working out what 3pm in Seoul means in San Francisco, and whether the London team is even at their desks then, is a small tax you pay repeatedly. Daylight saving makes it worse, and the fact that the US and Europe switch on different dates makes it worse still.
Convert one reference moment into local time across as many cities as you like. UTC offsets and daylight saving come from the IANA timezone database built into your browser, and each card shows whether the date rolls over and whether the local time falls inside business hours. Unix timestamp and ISO 8601 representations are one click away.
How to use
- Set the reference moment — Enter a date and time and specify which timezone that wall-clock time belongs to. Set to now fills in the current moment.
- Add the zones you care about — Use Add timezone and search by city or region. Your own zone plus UTC, New York, London and Tokyo are there by default; remove any card with the × in its corner.
- Read the cards — Each card shows local time, date, UTC offset and abbreviation (KST, EDT). A badge flags when the date differs from the reference zone, and a green badge marks business hours — useful when hunting for a meeting slot.
- Copy timestamps — The Unix timestamp and ISO 8601 values copy with one click, ready for a log query or an API parameter.
Frequently asked questions
Is daylight saving handled?
Yes. The browser's IANA timezone database supplies each region's rules for the specific date you entered.
New York is UTC−05:00 (EST) in winter and UTC−04:00 (EDT) in summer; change the reference date and you will see the offset and abbreviation change with it. South Korea has not observed daylight saving since 1988, so it stays at UTC+09:00 year round.
Can I use abbreviations like KST or EST in code?
Avoid them — they are ambiguous and overloaded.
CST is US Central Time (UTC−06:00), China Standard Time (UTC+08:00) and Cuba Standard Time. IST serves India, Ireland and Israel. Abbreviations also encode daylight saving state, so EST and EDT are different strings for one region and neither is a stable identifier.
In code, always use IANA identifiers such as Asia/Seoul and America/New_York, which carry the full rule set.
Does a Unix timestamp have a timezone?
No, and that is exactly its value.
A Unix timestamp counts seconds since midnight UTC on 1 January 1970. The same instant produces the same number anywhere on Earth. A timezone is a presentation rule applied when you show that number to a person.
So store UTC or timestamps in your database and convert at display time. Storing local wall-clock time breaks at daylight saving transitions, where an hour either does not exist or occurs twice.
Concepts worth knowing
UTC, GMT and offsets
UTC is the atomic-clock-based international standard; GMT is a historical name rooted in mean solar time at Greenwich. In practice the values coincide, but standards and code use UTC.
An offset is the difference from UTC. Korea is a fixed UTC+09:00, but India is UTC+05:30 and Nepal UTC+05:45 — code that assumes whole-hour offsets breaks in those regions.
The IANA timezone database
Identifiers like Asia/Seoul come from the IANA tz database, which records not only current rules but the full history of offset changes for each region.
When a government changes its daylight saving policy, the database is updated and operating systems and browsers pick it up. A server with stale tz data will compute future dates incorrectly, so if your system schedules far ahead, keeping tz data current belongs on your operations checklist.
Scheduling across continents
There is effectively no hour when Seoul, London and San Francisco are all at work. 9am in Seoul is midnight in London and 4pm the previous day in San Francisco.
The workable answers are to pick a slot that overlaps two regions — 5pm Seoul is 9am London — and rotate who takes the inconvenient hour, or to replace the meeting with asynchronous written collaboration. The business-hours badges here make it quick to see which pairings overlap at all.