The fortnight when everyone gets it wrong
The United States moves its clocks on the second Sunday in March. The European Union moves on the last Sunday. For roughly two weeks each spring, New York and London are four hours apart rather than the five that everyone has memorised. In autumn the same thing happens in reverse, for about a week, when the EU falls back before the US does.
It causes more missed calls between those regions than anything else, and no planner in this category warns about it. This one does: when the gap between two places on the date you have chosen differs from the gap a month either side, it says so above the ribbon.
The southern hemisphere adds another layer. Australia moves in the opposite direction to Europe, so the London-to-Sydney gap swings across a three-hour range over the year rather than sitting still.
Why a band instead of a list of clocks
A list of current times in each city answers a question nobody asked. What you actually want to know is which hours are simultaneously reasonable in several places, and that is a shape, not a number — the overlap between three bands.
So the output is a horizontal grid, one row per place, one cell per hour. Two things are encoded independently: brightness says when it is, and the hue on the row edge says whose row it is. Keeping those separate is what makes the overlap readable at a glance rather than something you have to work out.
An hour is marked as working for everyone only if it falls between eight in the morning and six in the evening in every place listed. If the best available slot has someone up at seven or on a call at nine at night, it is marked as such rather than presented as fine.
Nothing is stored
The places you pick are not saved anywhere and no account is offered. The whole thing is computed in your browser using the time zone database your browser already ships, which is also why it stays correct through daylight-saving transitions without anything being updated here.
Things people ask
Why does it say the clocks are out of step?
Because on the date you picked, two of your places have a different gap than they do for most of the year. The US and EU change clocks on different dates, so there are windows each year where the usual offset is wrong.
Can I add more than three places?
Yes, as many as you like. The ribbon scrolls sideways and each new place gets its own row.
What counts as working hours?
Eight in the morning to six in the evening, local. Six to eight and six to ten are shown as shoulder hours — usable, but at a cost to somebody.