Time-zone meeting planner

How the overlap is found

The planner takes the chosen date in the first person’s time zone and walks through it slot by slot, from local midnight to the next local midnight. For each slot it asks the browser what the local time is in every other person’s zone at that same instant, using the built-in Intl.DateTimeFormat API. A slot counts as available for a person when it starts and ends inside their working hours on a weekday in their own location. The rows where everyone is available are merged into the windows shown at the top.

No offsets are typed into this page. The browser carries the IANA time zone database (often called the tz or Olson database), the same record of every region’s offsets and daylight-saving rules that operating systems and calendar software use. That is why zones are named by a representative city, such as Europe/London, rather than by an abbreviation: “CST” can mean Central Standard Time in North America or China Standard Time, but America/Chicago means exactly one set of rules.

A worked example

On Wednesday 14 October 2026, New York is on daylight time (UTC−4), London is still on British Summer Time (UTC+1) and India does not change its clocks (UTC+5:30). The New York colleague works 08:00 to 17:00, London 09:00 to 17:30 and Bengaluru 10:00 to 19:00.

New York’s 08:00 is 13:00 in London and 17:30 in Bengaluru. Bengaluru’s day ends at 19:00, which is 09:30 in New York. So the shared window is 08:00 to 09:30 New York time, 90 minutes. Had the New York colleague started at 09:00, there would be no time at all when all three are working, and the planner would say so rather than suggest a slot outside someone’s day.

Move the date to 28 October and the window changes: the UK has returned to GMT (UTC+0) on 25 October but the US does not change until 1 November, so for that week New York is only four hours behind London instead of five.

When working hours are not enough

Working hours say when people could meet, not when they are free. Once you know the overlap, send the group an availability poll for those hours: each person marks the slots they can actually make, in their own time zone, and the results rank the times by how many people can come and name who cannot.

Making a hard overlap fair

• Rotate the inconvenience. If someone has to join early or late, alternate who it is rather than always asking the same region.
• Keep the overlap for what needs it. Decisions and discussion benefit from being live; status updates rarely do.
• Check again when the clocks change. A recurring meeting that fits in September can land outside someone’s day in November.

Where it stops being reliable

The results are only as current as the time zone data in your browser, which is updated with browser and operating-system releases. Governments occasionally change their rules at short notice, and an out-of-date browser can be wrong about a recently changed region. The planner also knows nothing about public holidays, lunch breaks or people who work different hours on different days, and it treats Saturday and Sunday as the weekend everywhere unless you tick the box. In regions where the working week is different, for example Sunday to Thursday, tick “count weekends” and check the weekday yourself.