Timezone Converter

Convert a date and time between IANA timezones and compare several zones at once, with day-change badges. Runs on your browser's built-in Intl data.

Runs 100% in your browser — nothing you enter leaves your device.

Time to convert

Converted times
  • UTC–
  • America/New_York–
  • Europe/London–

Zones you add are remembered in this browser (localStorage) — nothing leaves your device.

How to use

  1. Enter the moment you care about in Date and time, or press Set to now to drop in the current time down to the minute.
  2. Choose which zone that reading belongs to under In timezone. It defaults to your own, detected from your browser, but you can pick any zone as the source.
  3. Type a city or region into Add a timezone to compare — suggestions appear as you type — then press Add zone. Repeat for each zone you want to see, up to twenty.
  4. Read the results below. A +1d or −1d badge marks any zone that has rolled over to a different calendar date than your source.
  5. Copy one row with its Copy button, or take the whole comparison with Copy all. The ✕ beside a row drops that zone.

How it works

No timezone table ships with this page. Every offset is read from the IANA database your browser already carries, reached through the Intl.DateTimeFormat API. That API is built to turn an instant into a wall-clock reading for a given zone — but a converter needs the opposite: you type a wall-clock time and it has to find the underlying instant. The tool inverts Intl by reading the zone’s offset at roughly the right moment, subtracting it, then running a second refinement pass so daylight-saving boundaries land on the correct side.

Take 6:00 pm on 21 November 2026, entered as Australia/Adelaide. Adelaide observes daylight saving that month, so its offset is UTC+10:30. The converter subtracts it — 18:00 minus 10 h 30 m — to recover 07:30 UTC as the real instant. From there every target is a plain addition. Asia/Tokyo runs UTC+9 year-round, so it reads 4:30 pm the same day, no badge. America/Los_Angeles, where clocks fell back to UTC−8 on 1 November, reads 11:30 pm — but on 20 November, a day behind the source, so its row shows a −1d badge. Change the source date to July and the same cities shift, because the browser applies each zone’s offset as it stood on that date, not a fixed number.

Use cases & limitations

The everyday reach for this is scheduling: fixing a call time that lands sanely for a team split across three continents, or working out what “end of business Friday” in one office means for another. It is equally handy when a log line or ticket quotes a time in an unfamiliar zone and you want it in yours — pair it with the Unix timestamp converter when the source is an epoch number rather than a written date, or with the cron parser when you are reasoning about when a scheduled job actually fires.

The honest limit is that accuracy is only as fresh as your browser. DST rules are politics, not physics — governments change them, sometimes at short notice — and the offsets here come from whatever IANA data your browser was last updated with. A years-out-of-date browser can be wrong about a newly legislated transition. Two edge cases are resolved automatically: a time that never happens on a spring-forward night lands on the instant the clock actually reaches, and a time that happens twice on a fall-back night uses the first occurrence.

Privacy note

Conversion runs entirely in the page and makes no network requests — the timezone data is already inside your browser, so it works offline once loaded. The list of zones you add is saved in this browser’s localStorage under a single key, on this device only; it is never uploaded, and clearing your site data removes it.

Frequently asked questions

How does the converter know when daylight saving starts in each timezone?

It doesn't ship its own table — it asks your browser. Every browser bundles the IANA timezone database through the built-in Intl API, with past and scheduled DST transitions for every zone. When you convert a date, the browser applies the offset in force at that moment, so a July conversion for Sydney uses AEST (UTC+10) while a January one uses AEDT (UTC+11).

Why are timezones named after cities, like Australia/Sydney, instead of AEST or GMT+10?

Abbreviations are ambiguous — CST is used for China, Cuba and US Central time — and fixed offsets break the moment daylight saving flips. An IANA name identifies a region whose clocks have always agreed with each other, so Australia/Sydney stays correct all year while "+10" is only right for half of it.

What does the +1d or −1d badge on a row mean?

The same instant falls on a different calendar date in that zone than in your source zone. 8 pm Thursday in Los Angeles is already noon Friday in Tokyo, so Tokyo's row shows +1d. It is the detail that quietly ruins meeting invitations, which is why it gets a badge instead of a footnote.

What happens if I enter a time that doesn't exist, like 2:30 am on a spring-forward night?

When clocks jump from 2:00 straight to 3:00, 2:30 never happens in that zone. The converter resolves it the way calendar apps do — it lands on the instant the clock actually reaches (3:30 that morning) and shows a note so the adjustment isn't silent. For the opposite case, an ambiguous time that occurs twice when clocks fall back, it uses the first (pre-transition) occurrence.

What's the difference between UTC and GMT?

For converting a meeting time, nothing you will notice — both mean zero offset. Technically GMT is a timezone historically tied to solar time at Greenwich, while UTC is the atomic-clock standard all zone offsets are defined against. Both appear in the zone list; pick UTC when you want to be unambiguous in technical contexts.

Is my zone list saved anywhere, and does the tool work offline?

The zones you add are kept in your browser's localStorage under a single key, so the list is waiting next visit — on this device only, never sent to a server. Conversion itself makes no network requests at all: the timezone data is already inside your browser, so once the page has loaded everything works offline.