World Clock

Reading the board

Your own time sits at the top, with your zone and its offset from UTC underneath it. Below that is the board, ordered east to west, so the cities furthest ahead of UTC are at the top and the day runs downward. Each row carries the local time, the date there, the offset from UTC, and how far ahead or behind you it is.

Rows are shaded by local hour. A dimmed row means the middle of the night, a warm tint means early morning or evening, and a plain row means the working day. That shading is doing more work than it looks: most of the time somebody checks a world clock, the real question is not what the clock says but whether it is a reasonable moment to call, and a colour answers that faster than a number does.

When a city is on a different calendar date from you, the date is picked out in the accent colour and labelled. That matters more than people expect. Sydney is far enough ahead of London that for most of the London working day it is already tomorrow in Sydney, and a message sent on a Friday afternoon in one is landing on a Saturday in the other.

Daylight saving, and why the gap between two cities moves

The distance between two cities is not a constant. New York is usually five hours behind London, and for about two weeks each year it is four, because the United States and the United Kingdom do not change their clocks on the same date. The US moves on the second Sunday in March and the first Sunday in November; the UK and the rest of the EU move on the last Sundays of March and October. In the gaps, every rule of thumb anyone has memorised is wrong by an hour.

The southern hemisphere inverts the whole thing. When Europe is going into summer time, Australia is coming out of it, so the gap between London and Sydney swings across a range of three hours over the course of a year. And plenty of places do not observe daylight saving at all, including India, Japan, most of Africa, and Arizona but not the rest of the United States.

None of this needs handling by you. The board asks the browser for each city's offset at this exact moment, and the browser answers from the same time zone database your operating system uses. What it means practically is that you should not memorise a gap. Check it.

The offsets that are not whole hours

A surprising number of places sit on a half or quarter hour. India is UTC+5:30 for the entire country, which is a single zone spanning about 3,000 kilometres. Nepal is UTC+5:45, deliberately offset from India by fifteen minutes. Iran is +3:30, Afghanistan +4:30, Myanmar +6:30, and parts of central Australia are +9:30. The Chatham Islands, east of New Zealand, are +12:45.

These are shown properly here, which sounds like it should go without saying and does not: a good deal of scheduling software still assumes whole hours somewhere in its stack, and the failure shows up as a meeting invitation that is fifteen or thirty minutes out for exactly one participant.

UTC, GMT, and which one to use

UTC is Coordinated Universal Time, the reference every other zone is expressed against. GMT is Greenwich Mean Time, the older term, and for everyday purposes the two are the same. The difference is technical: UTC is maintained by atomic clocks and occasionally has a leap second inserted, while GMT is defined astronomically.

The practical trap is that GMT is also the name of a time zone that the United Kingdom is in only during winter. From late March to late October, London is on British Summer Time and is UTC+1, not UTC. Writing "10am GMT" in June is therefore ambiguous at best and wrong at worst, and it is the single most common time zone mistake in written communication. Write UTC and an offset, or write the city.

What this does not do

This board shows now. If you need to convert a specific time, rather than read the current one, the time zone converter takes a moment in one city and shows it in all the others, marks whether it lands inside working hours, and gives you a link you can send to everyone involved.

If you want to be told when a particular moment arrives rather than watch for it, the countdown counts to a date and can be shared the same way, and the alarm clock rings at a time of day.