Reading the board

The whole idea is that you stop converting times in your head and start looking down a column instead.

Columns are your day, not theirs

Every column on the board is one hour of your local day, midnight on the left through 23:00 on the right. A cell shows what that same moment looks like in the city on that row. Two cities are reachable at the same time when their cells in one column are both light.

This is the part ordinary world clocks get wrong when they line cities up by each city's own midnight. Those boards look tidier and tell you nothing, because a vertical slice through them is not a shared moment.

What the shading means

Daylight covers 07:00 to 17:59 local.
Dawn and dusk mark 05:00, 06:00, 18:00 and 19:00, the hours where a call is possible but rude to assume.
Early or late covers 20:00 to 22:59.
Deep night is 23:00 to 04:59.

The heavier line along the bottom of a cell marks conventional working hours, 09:00 to 17:59 local. It is drawn separately from the shading because the two answer different questions: whether someone is awake, and whether they are at work.

The vermilion line

The vertical line crossing every strip is the current moment, positioned to the minute. It moves on its own and the board refreshes every thirty seconds. Click any cell to pin a column and the readout above the board switches from the current time to that hour in every city you have added. Clicking again releases it.

The eleven zones of Russia

Kaliningrad to Kamchatka is a ten hour spread. The working day in Kaliningrad begins after the one in Kamchatka has already ended, so there is no common working window between the two edges of the country at all. The board shows that honestly instead of pretending a meeting for everyone is possible.

Russia abolished daylight saving in 2014, so the offsets of Russian cities do not move during the year. Berlin, London and New York do move, on different dates, and for those few weeks each spring and autumn the familiar gap stops being familiar. That is exactly when this board earns its keep.

Daylight saving is handled, not guessed

Offsets come from your browser's own time zone database through Intl.DateTimeFormat, so a city that has just moved its clocks is drawn correctly without anything being updated here.

A caveat. If your own machine is set to the wrong time zone, every row is wrong together and the board will look perfectly consistent while doing so. The offset next to each city name is there so you can sanity check one you know.

What it does not do

There is no scheduling, no invitations, no participant list, no sync across devices. The city selection is kept in your browser's local storage and never leaves it. If you clear site data it goes back to the default five.

Those omissions are deliberate. A tab that sits open on a second monitor all day should do one thing and cost nothing to glance at.