Time Zone Converter

Every time zone is described as an offset from UTC, so converting between two of them is a single subtraction: destination offset minus source offset, added to the clock time. If it is 12:00 in New York (UTC−5) and you want London (UTC+0), the difference is +5 and the answer is 17:00. Going the other way, subtract. From New York to Tokyo (UTC+9) the difference is +14, so noon in New York is 02:00 the next day in Tokyo — which is why the calculator flags a day rollover. The day note is the part people forget. A "3 pm Tuesday" call arranged from the US west coast is Wednesday morning for a colleague in Sydney or Tokyo, and a meeting invitation that carries only the time and not the date will be missed by exactly one day.

UTC — Coordinated Universal Time — is the reference every offset is measured against. It is not a time zone belonging to any country; it is a time standard, maintained from a worldwide ensemble of atomic clocks and kept aligned with the Earth's rotation. The odd abbreviation is a deliberate compromise: English would give CUT and French TUC, so the neutral UTC was adopted so that no language had the natural acronym. GMT, by contrast, is a time *zone* — the one the UK uses in winter — and also a historical standard that UTC superseded. The distinction becomes practical every spring: London observes British Summer Time from late March to late October, which puts it at UTC+1. So "London time" and "GMT" are the same only about half the year, and an invitation saying "3 pm GMT" in July is ambiguous. A useful discipline: UTC never changes for daylight saving, ever. Quote critical times in UTC and the ambiguity disappears entirely. This is why aviation, shipping, servers and international broadcasting all schedule in UTC and convert locally. UTC has also historically absorbed leap seconds, occasional one-second insertions that keep it within 0.9 seconds of the Earth's actual rotation. In 2022 the international metrology community resolved to phase the practice out by 2035, since irregular seconds cause more trouble for computer systems than the drift they correct.

The mental model of 24 neat one-hour slices is wrong, and scheduling software that assumes it breaks in interesting ways. Several large zones run on half-hour offsets: India is UTC+5:30, Iran is UTC+3:30, Afghanistan UTC+4:30, Myanmar UTC+6:30, and Adelaide and Darwin in Australia are UTC+9:30. Newfoundland in Canada is UTC−3:30. A few go further and use 45-minute offsets. Nepal sits at UTC+5:45, and the Chatham Islands off New Zealand at UTC+12:45. This matters because so much scheduling is done by counting hours. A call that is "9 am my time, so 6 pm yours" quietly loses half an hour when one end is in Mumbai and forty-five minutes when one end is in Kathmandu. The offsets in this converter are whole hours, so add the extra fraction manually for those destinations. Offsets are also political rather than geographical. China spans roughly five zones' worth of longitude and runs the entire country on one, UTC+8, which puts the far west several hours out of step with the sun.

The zones in this converter are standard offsets, with no daylight saving applied. That is the right default, but it is also the single biggest source of real-world mistakes, for a reason that is not obvious: countries do not switch on the same dates. The US moves to daylight time on the second Sunday in March; the EU and UK move on the last Sunday in March. For roughly three weeks in between, New York is on UTC−4 while London is still on UTC+0 — so the familiar 5-hour gap is temporarily 4 hours. The mirror image happens in autumn, when Europe falls back a week before the US does. The southern hemisphere inverts everything. Sydney observes daylight saving through the southern summer, from October to April, so the New York-to-Sydney difference swings by a full two hours over the course of a year and briefly passes through several values. And plenty of places do not shift at all: most of Arizona, all of Hawaii, and the great majority of countries near the equator, where the seasonal daylight variation is too small to be worth it. The practical rule is to convert for the specific date, not in general. Two cities do not have a fixed difference — they have a difference that depends on the calendar.

Because zones run from UTC−12 through UTC+14, the inhabited world covers about 25 hours at once — more than a single day. At any given moment there are two different calendar dates in use somewhere, and briefly three. The International Date Line is where the calendar rolls over, running roughly along the 180th meridian in the Pacific. It is deliberately not straight: it zigzags to keep island groups and national territories on a single date, which is why Kiribati stretches all the way to UTC+14 to avoid splitting the country across two days. Crossing it changes the date without changing the time. Fly west across it and you lose a day; fly east and you live the same date twice. Countries can move themselves across it too — Samoa and Tokelau shifted from the east side to the west at the end of 2011 to align their working week with Australia and New Zealand, and simply skipped December 30 that year. It did not exist for them. The authoritative record of who is on which offset, and when they change, is the IANA time zone database, updated several times a year as governments alter their rules. Operating systems and programming languages pull from it, which is why a phone that has not been updated in years can quietly show the wrong local time for a country that changed its policy.