Free Time Zone Converter

The Complete Guide to Time Zones — How They Work, Why They're a Mess, and How to Convert Them Without Going Insane

Time zones are one of those topics that seem trivially simple until you actually try to schedule a meeting between London, Mumbai, Singapore and Sydney during the week when the UK switches to summer time but Australia is still on winter time. Then they become a minefield of off-by-one-hour errors, missed calls, and apologetic Slack messages reading "sorry, I thought 3pm was your 3pm." This guide explains how the system actually works, why it's so prone to mistakes, and how to use the LazyTools converter to handle every scenario without doing UTC arithmetic in your head.

The basics — UTC and the offset system

Every time zone in the world is defined as an offset from UTC (Coordinated Universal Time), the modern global time standard maintained by atomic clocks. The offset can be positive (zones east of Greenwich) or negative (zones west). India is UTC+5:30, meaning when it's noon UTC, it's 5:30pm in Mumbai. Los Angeles is UTC−8 in winter (PST) and UTC−7 in summer (PDT). The total range of standard offsets runs from UTC−12 (Baker Island) to UTC+14 (Kiribati's Line Islands) — a 26-hour spread, larger than 24 hours because the international date line zigzags around the Pacific to keep certain countries on a single calendar date.

Why the maths in your head goes wrong

Mental time-zone arithmetic seems easy until you hit one of the dozen things that can go wrong. Daylight saving time changes the offset for half the year in some zones but not others — and the start and end dates of DST are different in the UK, the EU, the US, Australia and New Zealand. The result is a "DST gap" period each spring and autumn when zones that are normally exactly 5 hours apart are temporarily 4 or 6 hours apart for two weeks. Half-hour and quarter-hour offsets trip people up: India is UTC+5:30, Newfoundland is UTC−3:30, and Nepal is UTC+5:45. Day boundaries are easy to forget: 11pm Friday in New York is 4am Saturday in London — converting Friday to Saturday is the easiest mistake to make on a calendar invite. Zone abbreviations are ambiguous: "CST" can mean US Central Standard Time, China Standard Time, or Cuba Standard Time. "IST" can mean India Standard Time or Irish Standard Time. "BST" can mean British Summer Time or Bangladesh Standard Time. A converter that works on the IANA identifier (like Asia/Kolkata or Europe/London) sidesteps all this ambiguity.

The IANA time zone database — the source of truth

Behind every reliable time zone tool, every operating system, every modern programming language and every web browser is the same data source: the IANA Time Zone Database (sometimes called the Olson database after its founder Arthur David Olson). It catalogues every time zone in the world by a region/city identifier (Europe/London, America/New_York, Asia/Tokyo) and stores the historical and current rules for offsets, DST transitions, and political changes — including changes that haven't happened yet but are scheduled. When Saudi Arabia changes a holiday, when a country adopts or drops DST, when a province moves between zones, the IANA database is updated and the world's clocks update with it. The LazyTools converter uses your browser's built-in copy of this database via the standard Intl.DateTimeFormat API, so the conversion is always as accurate as your browser is up-to-date.

Daylight saving time — the bane of meeting schedulers

About 70 countries observe some form of daylight saving time. The European Union shifts on the last Sunday of March (clocks forward to summer time) and the last Sunday of October (back to winter time). The United Kingdom follows the same dates as the EU since the rules were inherited and never changed. The United States and Canada shift on the second Sunday of March and the first Sunday of November — different dates from the EU, which creates a two-week window each spring and autumn when the time difference between London and New York is 4 hours instead of the usual 5. Australia and New Zealand are in the southern hemisphere, so their DST is October to April (their summer), the inverse of the north — and only some Australian states observe it (Queensland, Northern Territory and Western Australia do not). Most of Asia, Africa and South America do not observe DST at all. India, China, Japan, Singapore, all the Gulf states, and most of South America stay on a single offset year-round. The complexity of remembering which zones are currently on which version is exactly why mental arithmetic fails and a date-aware converter wins.

Half-hour and quarter-hour zones — the awkward cases

Most time zones are offset from UTC by a whole number of hours, but a handful use half-hour or quarter-hour offsets, usually for political or geographic reasons. India and Sri Lanka are UTC+5:30. Nepal is UTC+5:45 — uniquely chosen so that the meridian of Mt Gaurishankar (a sacred peak) sits at noon. Iran is UTC+3:30 (UTC+4:30 with DST). Afghanistan is UTC+4:30. Myanmar is UTC+6:30. Newfoundland (Canada) is UTC−3:30. Parts of Australia use UTC+9:30 (Adelaide, Darwin) and UTC+10:30 (with DST). The Chatham Islands (NZ) are UTC+12:45. The half-hour zones cause real problems for software that assumes integer hour offsets — the kind of bug that causes calendar invites to arrive at 7pm instead of 7:30pm. The IANA database handles them correctly; sloppy mental arithmetic does not.

How to schedule international meetings without losing your mind

For two-zone calls, the quick converter is enough: pick source, pick target, pick time, done. For three or more zones, switch to meeting planner mode — it solves a different problem. The planner draws every zone as a horizontal bar of 24 one-hour cells, colour-coded by what each hour means in that zone. Indigo cells are business hours (9am to 6pm in the local zone). Deeper indigo cells are prime hours (10am to 5pm — when people are most reliably available). Light grey cells are early or late but workable in a pinch. Dark cells are night and should be avoided. The visual lets you see at a glance which columns have indigo across all rows — those are the slots that work for everyone. For a London ↔ New York meeting, the obvious slot is 2pm London / 9am New York — both inside business hours. For a London ↔ Mumbai meeting, 2pm London / 7:30pm Mumbai works for an English afternoon and an Indian evening. For London ↔ Sydney, the overlap is brutally narrow: 7am London / 6pm Sydney is roughly the only workable slot, and even that pushes Sydney to the very edge of the workday. Knowing this in advance saves a lot of "let me check with the Sydney team" delays.

Why time zones exist at all

Until the late 19th century, time was local — every town set its clocks by the position of the sun, so a town 50 km east of you might be 2 minutes ahead. This worked fine when travel was slow and communication was face-to-face. The arrival of railways made it untenable — railway timetables needed a standard reference, and trains running between cities couldn't deal with each station having its own clock. In 1840 the Great Western Railway in the UK adopted "London time" (GMT) for all its operations, regardless of where the station was. Other railways followed, and gradually entire countries standardised on a single time. The international time zone system was formalised at the International Meridian Conference in Washington in 1884, which adopted the Greenwich meridian as the global reference and divided the world into 24 zones of one hour each. The system has been politically tweaked, fragmented and patched ever since — half-hour zones appeared, DST was invented (originally as a wartime fuel-saving measure in 1916), countries hopped between offsets, and the whole arrangement has accumulated 140 years of historical and political cruft. Every modern time zone tool is essentially a database of all these decisions plus the rules for applying them.

Common conversions people search for

Some time-zone conversions are searched for so often they have their own folklore. EST to IST — US Eastern to India Standard Time, a 9.5-hour difference (10.5 in summer when EDT is in effect). New York 9am = Mumbai 6:30pm in winter, 7:30pm in summer. PST to GMT — Pacific to Greenwich. LA 9am = London 5pm in winter, 4pm in summer (the gap shrinks by an hour because both zones DST but on different dates). IST to PST — Mumbai to Los Angeles. The full 12.5-hour difference in winter (13.5 in summer) makes this the most challenging common pair to schedule. London to Tokyo — 9 hours in winter, 8 in summer. Singapore to London — 8 hours in winter, 7 in summer. Dubai to New York — 9 hours in winter, 8 in summer. The recurring theme is that nearly every common conversion changes by an hour twice a year because of DST — and gets it right or wrong is the difference between a meeting that happens and a meeting that gets missed.

Pair this with the workweek calculator

Knowing what time it is somewhere isn't quite the same as knowing whether anyone's actually working. A 10am Friday slot in your zone might be 10am Saturday in theirs — and Saturday is the weekend in most countries, but Friday is the weekend in Saudi Arabia, and the UAE only switched to Saturday-Sunday in 2022. For deadline calculations that mix time zones with working calendars (SLAs, project deliverables, contract notice periods), pair this converter with the LazyTools Workweek Calculator. It handles the business-day count with country-specific public holiday databases and configurable workweeks (Mon–Fri, Sun–Thu for the Middle East, or any custom combination), so you can answer questions like "what's the deadline if it's 30 working days from a Mumbai start date in IST?" without getting tangled up in either the calendar or the clock.

Frequently asked questions