Two things go wrong, and a converter shows you neither
Working out that 16:00 in London is 01:00 in Sydney is the easy half. Every converter does that. The two things that actually cause the problem are the ones that never appear.
1. The date is different
It is 01:00 tomorrow in Sydney. A meeting booked into the wrong day is a different failure from a mistimed one and a much worse one: everybody turns up, one person is not there, and nobody can work out why until somebody looks at the invitation properly.
So this checks every hour of your working day and says which places roll over to another date and from what time. If you take one thing from the page, it is that an invitation should have the date written out in it, not just the time.
2. The gap between two places is not constant
Nearly everybody carries a fixed number in their head. London is five hours ahead of New York. It is, for about 49 weeks a year.
- The United States moves to daylight saving on the second Sunday in March.
- Europe moves on the last Sunday in March.
- For the roughly three weeks between them, London to New York is four hours, not five.
- They change back a week apart in the autumn as well, so it happens twice a year.
A recurring meeting set up during one of those windows is wrong for the other eleven months. The tool looks a fortnight either side of your date and says when a change is coming.
The southern hemisphere moves the opposite way
This one catches people who already know about the first two.
Sydney is on daylight saving during the northern winter. So London to Sydney is 11 hours in January, 10 in July, and 9 for the few weeks when one has changed and the other has not. Three different answers to "how far ahead is Sydney", all correct, depending on the date.
And a lot of places never change at all
India, China, Japan, Singapore, most of Africa, and within countries that do change: Arizona, Queensland, the Northern Territory and Western Australia.
Which produces the least intuitive case of the lot. When your clocks change and theirs do not, the gap between you moves even though nothing happened at their end. Your 09:00 call becomes their 13:00 instead of their 14:00, and the person who has to adjust is you.
Why there is no calendar file to download
Because it would be wrong in exactly the way this page exists to prevent. Our calendar builder writes timed events as floating, with no zone attached, and a floating time is read by every calendar as the recipient's own local time.
Send that to five countries and five people get a meeting at 13:00 their time, which is five different meetings. Rather than ship a file that quietly does the one thing the tool warns about, the copy button gives you every city with its time, its date and its offset, which is what belongs in the invitation anyway.
When nothing suits everybody
With London, San Francisco and Sydney in the same call there is no hour inside all three working days. That is arithmetic, not a failure to look hard enough, and the tool says so rather than offering you the least bad hour as though it were a solution.
The useful response is to decide who is inconvenienced and to rotate it, rather than defaulting permanently to whoever happens to be booking. Everything here runs on your own machine and nothing is uploaded.
Common questions
Why does it keep telling me the date is different?
Because that is the mistake that actually happens. Everybody agrees "four o'clock", the invitation goes out, and one person has it in the diary on the wrong day. At 16:00 in London it is 01:00 in Sydney and that is tomorrow morning, not tonight. The tool checks every hour of your working day and says from which hour each place rolls over.
I thought London and New York were always five hours apart.
They are for about 49 weeks a year. The United States moves to daylight saving on the second Sunday in March and Europe on the last Sunday in March, so for roughly three weeks the gap is four hours instead of five. They change back a week apart in the autumn too. Anything you set up inside those windows is wrong for the rest of the year.
Why did my recurring meeting drift?
Almost certainly a clock change at one end. If a series was created as "14:00 London" it stays at 14:00 London, so it moves for everyone else when their clocks change and yours do not. This tool warns when any of the places you have chosen changes its clocks within a fortnight of the date you are planning.
Does the southern hemisphere really move the other way?
Yes, and it catches people out constantly. Sydney is on daylight saving during the northern winter, so the London to Sydney gap is 11 hours in January, 10 in July, and 9 for a few weeks in between when only one of them has changed. Three different answers to "how far ahead is Sydney" and all three are correct.
Which places do not change their clocks at all?
Most of Asia and Africa, all of India, China, Japan and Singapore, and within countries that do change: Arizona, Queensland, the Northern Territory and Western Australia. When your clocks change and theirs do not, the gap moves even though nothing happened at their end, and the adjustment is entirely yours to make.
Why is there no calendar file to download?
Because it would be wrong in exactly the way this tool exists to prevent. Our calendar builder writes times as "floating", with no time zone attached, and every calendar application reads a floating time as the recipient's own local time. Everyone would get the meeting at 13:00 their time. The copy button gives you the times and dates for every place instead, which is what actually goes into an invitation.
Where do the time zone rules come from?
Your browser. It carries the full IANA time zone database and the browser vendor keeps it current, which matters because governments change these rules with only a few months notice. A copy bundled into a web page would be out of date the first time somebody legislated.
Nothing suits everybody. What now?
Decide who is inconvenienced and rotate it, rather than defaulting to whoever booked the meeting. With London, San Francisco and Sydney in the same call there is genuinely no hour inside everyone's working day, which is a scheduling fact rather than a failure to look hard enough.