How this converter works

Converting a time between two places sounds like adding an offset, and that is exactly the assumption that makes most converters wrong twice a year. An offset is not a property of a place — it is a property of a place at an instant. New York is five hours behind UTC in January and four hours behind in July, and the switch happens at a moment defined by law, not by a formula.

So this tool never adds a number. It takes the wall clock you typed, works out which actual instant that reading corresponds to in the source zone, and then asks each destination zone what its clock read at that same instant. Everything downstream — the date, the day shift, the offset label — falls out of that single instant rather than being calculated separately, which is why the parts cannot disagree with each other.

Where the offsets come from

They come from your own browser. Every modern browser ships a copy of the IANA Time Zone Database — the same dataset operating systems, airlines and booking systems use — and exposes it through the standard Intl.DateTimeFormat interface. This page reads offsets out of that, rather than shipping a table of its own.

That is a deliberate trade. A bundled table would be identical for everyone and would start rotting the day it shipped; your browser's copy updates with your browser. The cost is stated honestly in the limitations below: if your device's data is old, this page inherits that, and it has no way to detect it.

Turning a wall clock into an instant

This is the step where converters go wrong, because the relationship is not one-to-one. A wall clock reading can correspond to two instants, or to none.

The method here is to probe a window of a day either side of the reading, take one candidate per distinct offset found, and then verify each candidate by converting it back— a candidate only counts if the zone genuinely renders it as the reading you asked for. The number of candidates that survive is the answer: one is ordinary, two means the reading happens twice, zero means it never happens at all. Counting survivors is what makes the ambiguous cases detectable rather than merely unlucky.

Where the reading is ambiguous, the default matches the convention used by the JavaScriptDate object and by the iCalendar standard that calendar applications are built on, so an answer here agrees with the invitation in your calendar rather than quietly differing by an hour. You can always pick the other one — the button is right there in the result.

Three worked examples

An ordinary day. 09:00 in New York on 15 June is 14:00 in London the same day. New York is on UTC−04:00 in June and London on UTC+01:00, five hours apart — but only in summer. In January the same pair is five hours apart too, and in the two weeks between the US and UK clock changes it is four. The tool never assumes which.

The hour that happens twice. Ask for 01:30 in New York on 1 November 2026. Clocks go back that morning, so 01:30 occurs once on UTC−04:00 and again an hour later on UTC−05:00. Both are real times; they are an hour apart in every other place on earth. The tool shows one, says plainly that it happens twice, and offers the other — labelled byoffset, never by abbreviation, because abbreviations are ambiguous and because Ireland's summer time is legally defined the opposite way round to everyone else's.

The two-day jump. Convert an early-morning time in Kiritimati to Midway. The legal range of offsets runs from UTC+14:00 to UTC−12:00 — twenty-six hours — so a single conversion can land two calendar days away. Any converter that shows a "(+1 day)" chip at most is wrong here, and this is the most consequential kind of wrong a converter can be: it is how people miss a meeting by a day and never find out why.

Sources

IANA Time Zone Database— the authoritative record of every zone rule and every change to one.
MDN — Intl.DateTimeFormat— the interface through which your browser exposes that data.
WCAG 2.2 — the accessibility standard this page is built and tested against.

Questions people actually ask

Why does my browser show a different time for a city than another site does?

Almost always because one of the two has older time-zone data. Countries change their rules — sometimes with a few weeks' notice — and every device, browser and website updates on its own schedule. If a place recently changed its clocks or abolished daylight saving, expect disagreement for a while. Where it matters, check the government source for that country.

What does "this time happens twice" mean?

When clocks go back, the hour before the change repeats. A reading like 01:30 occurs, then occurs again sixty minutes later on the new offset. Both are genuine, and they are different instants — so a meeting set for "01:30" that morning is ambiguous until somebody says which. It is a real scheduling hazard, not a curiosity, which is why this page surfaces it rather than choosing silently.

And "this time doesn't exist"?

The mirror image. When clocks jump forward, the skipped readings never occur — in most of the United States on the spring change, there is no 02:30 at all. Rather than pretend otherwise, the tool tells you what the clocks did and shows the nearest real time, with a button for the one on the other side of the gap.

Why do you show UTC+01:00 instead of BST?

Because abbreviations are ambiguous and inconsistent. IST means India Standard Time, Irish Standard Time and Israel Standard Time; CST covers US Central, China and Cuba. They are also language-dependent — the same zone abbreviates differently depending on the browser's language settings, and most zones in the world have no letter abbreviation at all and fall back to an offset anyway. An explicitUTC±HH:MM means exactly one thing everywhere. Where a genuine abbreviation exists, it is shown alongside the offset, never instead of it.

Why does a city I know is in one country show a different country's zone name?

Time zones are not countries. The database merges places whose clocks have agreed since 1970, so Oslo, Stockholm and Copenhagen all share a single zone identifier with Berlin, and northern Vietnam shares one with Bangkok while the south has its own. The city and country shown here are always the real ones; the underlying identifier is whatever the database uses.

How places are named here

Place and country names come verbatim from Unicode CLDR and the IANA Time Zone Database, and this site takes no position on any territorial claim. A territory appears in the list because IANA's country file carries the code — inclusion is a fact about data completeness, not recognition — and requests to change a name are answered by pointing at CLDR rather than by hand-editing the table. There are no maps, no flags and no borders anywhere on this site, deliberately and permanently. We quote the time zone database's own header on the point: “It is not intended to take or endorse any position on legal or territorial claims.” The full policy is on the about page.

Is anything I type sent anywhere?

No. The whole conversion runs in your browser, and your chosen places are saved in this browser's local storage so they are still here next time. Share links carry their state in the part of the URL after the #, which browsers never transmit to a server — so a link you share does not put your plans in anyone's server logs.

Limitations, stated plainly

Your device's time-zone data can be out of date, and this page cannot tell. Old Android versions and locked-down corporate machines are the usual cases. For a recently-changed jurisdiction, verify against an official source.
History before 1970 is approximate for a number of zones. The database deliberately does not model every pre-1970 local rule, so distant historical conversions should be treated as indicative.
Some historical offsets are not whole minutes. Monrovia ran 44 minutes and 30 seconds behind UTC until 1972. Where that happens, the offset is shown to the second rather than rounded into something tidier and wrong.
Future dates assume today's rules. If a country changes its law between now and the date you asked about, the answer changes with it. This is a limit of prediction, not of the tool.

Reviewed 2026-08-09. The date and time engine behind this page is tested against a reference implementation across three browser engines and three releases of the time-zone database.