Time Zone Converter

Why this is harder than adding hours

Time zones look like simple offsets from UTC and are not. Roughly a third of the world observes daylight saving, on different dates, in opposite directions depending on hemisphere. Some zones are offset by 30 or 45 minutes rather than a whole hour. Governments change the rules, sometimes at short notice.

This converter uses the browser's own IANA time zone database through the Intl API, which is why it gets the awkward cases right. It resolves the offset for the date you selected, not for today — so a meeting in January and a meeting in July between London and New York produce different answers, as they should.

The awkward offsets

China is the extreme case of political rather than solar time: the whole country runs on Beijing time, so in the far west of Xinjiang the sun can rise close to 10 am by the clock.

Daylight saving is not synchronised

This is where scheduling actually breaks. The US and Europe do not switch on the same dates — the US moves in March and November, the EU in late March and late October — so for a couple of weeks each year the usual gap between New York and London is an hour different from normal. A standing weekly call set up in February drifts in March and drifts back in April, and nobody notices until someone misses it.

The southern hemisphere is on the opposite cycle: when Europe springs forward, Australia is falling back, so the gap between London and Sydney varies by two hours across the year rather than one.

Plenty of places do not observe it at all — most of Asia, Africa and South America, plus Arizona and Hawaii within the US. Several countries have abolished it recently. The EU voted in 2019 to end seasonal clock changes and has not implemented it, so it continues.

Scheduling across zones without errors

State the time zone explicitly and use the city name rather than an abbreviation. Abbreviations are genuinely ambiguous: CST is Central Standard Time in North America, China Standard Time, and Cuba Standard Time, and they are not close together. IST is Indian, Irish, and Israel Standard Time.

Where a meeting matters, quote UTC alongside the local time. For recurring meetings across hemispheres, expect the relative time to shift twice a year and agree in advance whether the anchor is one city's clock or the other's — that decision is what determines who gets moved.

Business hour overlap

The practical constraint on distributed teams is not the offset itself but the overlap. India and the US west coast share almost no working hours — 9 am in Los Angeles is 9:30 pm in Mumbai. London to New York gives a comfortable three to four hours in the afternoon. London to Singapore gives roughly the same in the London morning. Sydney to San Francisco is the hardest common pairing, with a workable overlap only very early or very late for one side.

When picking a recurring slot, work out the overlap first and place the meeting in the middle of it rather than at an edge, so a daylight saving shift does not push it out of hours entirely.

Does this handle daylight saving?

Yes, and for the date you select rather than for today. It uses the browser’s built-in IANA time zone database, which knows the actual transition dates for each zone including historical ones. So converting a January time and a July time between London and New York gives different results, correctly — the two regions switch on different dates, so the gap between them is not constant even in the weeks around the changeover.

Why do some time zones have 30 or 45 minute offsets?

Because zone boundaries are political decisions, not geometric ones. India uses UTC+5:30 as a single zone for a country spanning nearly 30 degrees of longitude, splitting the difference. Nepal uses UTC+5:45, partly to be distinct from India. Newfoundland is UTC−3:30, and the Chatham Islands UTC+12:45. Any assumption that offsets are whole hours will eventually produce a wrong answer.

What is the difference between UTC and GMT?

For everyday use they are interchangeable, both being the zero reference. Technically UTC is a time standard maintained by atomic clocks and adjusted with leap seconds, while GMT is a time zone based on the mean solar time at the Greenwich meridian. UTC is the correct term for technical and scheduling use. Note that the UK is not on GMT year round — it uses British Summer Time, UTC+1, from late March to late October.

Why are time zone abbreviations ambiguous?

Because they were never standardised and many are reused. CST means Central Standard Time in North America, China Standard Time, and Cuba Standard Time. IST is Indian, Irish and Israel Standard Time. BST is British Summer Time and also Bangladesh Standard Time. For anything that matters, name the city or give the UTC offset — an abbreviation alone can be several hours out.

How do I schedule a recurring meeting across hemispheres?

Decide in advance which city’s clock is the anchor, because the relative gap will change twice a year — northern and southern hemispheres shift in opposite directions, so London to Sydney varies by two hours across the year. Place the meeting in the middle of the overlap in working hours rather than at an edge, so a daylight saving shift does not push it outside the working day for one side entirely.