Timezone Working Hours Overlap
Find the working-hour overlap between two time zones on a given day.
Written and maintained by Paul Clark, Redmoon Software · Rules last verified
These calculators are for informational purposes only and do not constitute legal, financial, or professional advice.
How the Timezone Working Hours Overlap works
The Timezone Working Hours Overlap calculator finds the hours on a given day when two offices are both inside their working window. Pick the date, your time zone, the other party’s time zone, and the working hours both ends keep, and it returns the overlap as a count of hours plus the window expressed in both local clocks — so you can read your own calendar and theirs from the same answer.
The overlap is computed hour by hour against real time-zone data rather than by subtracting UTC offsets, which matters more than it sounds. Daylight saving shifts the answer twice a year in each hemisphere, and because the northern and southern transitions happen at different times, a London–Sydney overlap can change without either party changing their hours. Zones on half-hour and three-quarter-hour offsets — Mumbai, Adelaide, Kathmandu — break naive arithmetic entirely. Because the calculation is anchored to a specific date, the result reflects the offsets actually in force that day, not an annual average.
A zero result is a real answer, not a failure. London and Sydney genuinely share no working hour on a standard nine-to-five, and the calculator says so plainly rather than returning an empty range, because knowing that a pairing has no overlap is what tells you to stop hunting for a meeting slot and design an asynchronous handover instead. The tool also suggests a single best hour from the middle of the overlap, which is usually a better meeting slot than either edge — the first and last hours of an overlap tend to be someone’s arrival or someone’s commute. What it does not carry is holidays: an hour can be inside both working windows and still be a public holiday at one end, so check the date against both calendars before sending the invitation.
Worked example
On Wednesday, 3 June 2026, London and New York on a nine-to-five both-ends pattern share three hours: 14:00–17:00 in London is 09:00–12:00 in New York. Swap New York for Sydney and the overlap is zero — Sydney’s working day has finished before London’s begins. Run Singapore against Sydney instead and there are six hours, 09:00–15:00 Singapore against 11:00–17:00 Sydney. Three pairings, three completely different collaboration models, from the same eight-hour working day at every site.
Frequently asked questions
What does a zero-hour result mean?
That the two zones genuinely share no working hour on that date under the window you set — London and Sydney on a standard nine-to-five is the classic example. It is a real finding rather than an error, and the right response is usually to stop looking for a meeting time and design around it: a written handover at the end of each side’s day, or one side agreeing to shift an hour or two. Widening the working window in the tool shows you exactly how much shift would be needed to buy an hour.
Does it handle daylight saving?
Yes. The overlap is computed against real time-zone rules for the specific date you enter, so the answer reflects the offsets actually in force that day. This is why the date field matters: a London–New York overlap is stable across most of the year but changes during the two or three weeks when one has switched and the other has not, and a London–Sydney pairing swings by two hours between June and December.
Does it know about public holidays?
No. It answers the clock question only. An hour can sit squarely inside both working windows and still be a public holiday at one end, which makes it useless for a meeting. Check the date against the holiday calendars at both sites before you send the invitation — the Working Days Between Two Dates calculator will do that part for either country.
Does it account for daylight saving?
Yes — it uses the time-zone database in your browser, so the overlap is computed for the actual offsets on the date you choose. That is why the answer can change from one week to the next around a transition, and why it is worth checking the specific date rather than assuming a pattern holds across a season.
What this calculator does not do
A limitation you know about costs far less than one you find after the deadline. These are the specific things this tool cannot work out for you.
- It computes the overlap of two working windows on a single date. Both parties are assumed to keep the same hours every working day.
- Public holidays in either zone are not deducted from the overlap — a working hour is not the same as a day someone is at their desk.
- Overlaps around a daylight-saving transition change from one week to the next; check the specific date rather than assuming the pattern holds.
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