The problem is not the maths, it is the assumptions
Working out that Karachi is five hours ahead of London is easy. What catches people out is everything around it: the gap changes twice a year, it changes on different dates in each country, and the two sides often disagree about what counts as a reasonable hour.
This planner sidesteps all of that by asking your browser what the local wall clock says at each moment, rather than adding fixed offsets. Daylight saving, half-hour zones and rule changes are handled because the underlying data ships with your device and is kept up to date by your operating system.
Daylight saving is the part that breaks calendars
Between mid-March and early April, the United States has already moved to summer time and Europe has not. For those weeks London and New York are four hours apart instead of the usual five. The same thing happens in reverse in autumn. A recurring meeting booked in local time will silently shift for one side and stay put for the other.
It gets stranger further out. India and Pakistan do not observe daylight saving at all, so their gap to Europe changes twice a year without anything changing at their end. Australia and New Zealand run the opposite way to the northern hemisphere, so a European and an Australian can find their difference swinging by two hours across the year.
The practical advice: for a recurring call, pick the date range you care about and check it here at both ends of the year. If the overlap is only just wide enough in January, it may not exist in July.
Reading the strips
Every row is the same 24 hours of the same day, laid out left to right. A green block means that person is inside their working window at that moment; grey means they are not. The Everyone free row at the bottom lights up only where every green block lines up.
Because the rows share one timeline, a city far enough east or west will show its date rolling over mid-row. That is not a glitch — it is the thing that makes scheduling across the Pacific confusing, made visible. A 9am meeting in Auckland genuinely is a Thursday afternoon in Los Angeles.
When there is no overlap at all
Sometimes the honest answer is that nine to five does not intersect. Karachi and Honolulu are fifteen hours apart; there is no hour of the day when both are at work. The tool says so rather than offering you a compromise that is not really there.
When that happens you have three options, and it is worth being deliberate about which one you choose. Widen the window and accept that somebody starts early or finishes late. Rotate whose day gets disrupted, so it is not always the same person. Or stop meeting synchronously and write things down instead — for genuinely distributed teams the third option is usually the right one, and the meeting was probably an email.
Questions people ask
Where does the time zone data come from?
From your own device. Browsers ship the IANA time zone database and expose it through the Intl API, which is what this page uses. There is no lookup service behind it, which is why it works offline and why nothing about your meeting leaves your machine.
Why does it show a city name and not the country?
IANA zones are named after a representative city rather than a country, because borders and rules change more often than the zones do. Pick any city in the same zone and the answer is identical — Europe/Berlin and Europe/Madrid are the same clock.
Can I plan around someone’s weekend?
Tick Skip weekends on their row and Saturday and Sunday in their local time are treated as unavailable. This matters more than it sounds: the weekend is not the same 48 hours everywhere, and in much of the Middle East the working week runs Sunday to Thursday.
Does it handle half-hour zones?
Yes. India is +05:30, Nepal is +05:45, and parts of Australia are +09:30. The offsets shown are exact rather than rounded to the nearest hour.
Is anything sent anywhere?
No. The calculation happens in your browser and the cities you enter stay on your device. There is no request from this tool at any point. The site’s theme loads its web fonts from Google on every page, as most WordPress themes do, and that is separate from this tool.