The clock board

A grid of hours. Every column of it is one instant and every row is one city's clock, so a finger run across the grid is a single moment read in several places at once. The hours are shaded by what they are locally: the small hours dark, the working day pale, the evening between them. A bar lies across the grid at the selected time and can be dragged along it or widened into a meeting, and a line marks where the clock actually stands now.

Every column of the board is the same instant. Run a finger down one and you are looking at one moment in five cities at once, which is the thing a single converted time cannot show you: not what three o'clock in London is in Tokyo, but where the two days sit against each other, and therefore which hours you could actually both be awake for.

The bar across the board is the time you have selected. Drag it anywhere, taking hold of the knob at its head when the hours run across, or click a cell to send it there. Drag either end and it becomes a stretch of time rather than a moment, which is what a meeting is: set it to an hour and every row tells you what that hour is called where they are. Hold alt while dragging to leave the quarter-hour steps behind. The arrow keys do the same thing without a pointer, an hour at a time with shift held, a whole day with Page Up and Page Down.

The shading is the local hour and nothing else. The pale band is the working day, the two blues either side of it are the early morning and the evening, and the dark is the small hours. It deliberately does not shade a good time to call. That would be a judgement about somebody else's Tuesday, and a board that made it would be wrong for shift work, wrong for Friday afternoon, and wrong for everybody in a country whose weekend is not Saturday. You can see that it is three in the morning there. What that means is yours to decide.

The letters beside each city are the abbreviation its clock is written down in: JST in Tokyo, BST in London in summer and GMT in winter, EDT and EST in New York. They are a reading at the selected instant and not a standing fact about the place, so they change as you drag the bar across a morning when the clocks move. Most of the world has no such letters, and there the badge is the offset instead, written UTC+5:30. That is not a gap in the table. It is how those clocks are named.

Two places on one board are often on different days, and that is the commonest thing a converted time hides. When the working day starts in Tokyo it is the previous evening in New York, so the two rows are printing different dates for the same instant, and the cell where a city's date turns carries the date instead of the hour to say so. The day named above the board is home's, the row marked with the house. A board six hours either side of midnight has two dates on it and the control above has room for one, so it names the one every other reading is measured against.

The number beside each city is how far it sits from home, and it is not a constant. London and New York are five hours apart for three weeks in March and one in October, not the four everybody quotes, because the two countries move their clocks on different weekends. The board shows the five. The southern hemisphere moves the other way in the other half of the year, so Sydney and London swing between nine hours apart and eleven, and a rule of thumb for that pair is wrong four months in twelve.

Twice a year a day is not 24 hours long, and the board draws that rather than tidying it away. On the morning a clock springs forward, that city's row runs from one o'clock straight to three, because two o'clock is an hour that did not happen, and the cell that replaced it is marked. On the morning it falls back, the row reads one o'clock twice, because it was one o'clock twice, and both cells hold real instants an hour apart. The board is 23 cells wide on one of those days and 25 on the other. Anything that drew 24 on both would be inventing an hour on one and swallowing one on the other, and every other row would quietly stop lining up. Nothing about any of this is written down in the calculator. Every offset comes from your own browser's zone data at the instant in question, which is why a country changing its rules is not a release here.

The switch marked 24-hour and am / pm changes only how the clock is written, never what it means. It starts on whichever your browser says your part of the world uses. The other switch turns the board: hours down the page, which is the phone's only shape, or hours across it, which fits more of the day on a wide screen. Both choices are remembered, and so are the places you put on the board.

The conversion answers on the calculator draw this same board underneath themselves, set to the places and the instant of whatever was asked about, which is why a link out of one of them opens this page already made. Ask for 3pm tokyo in london and the board you get here is the pair that answer converted. The link at the foot of this board carries the same thing the other way: it holds the places, the selection, the day and both switches, so a board sent to somebody opens as the board you were looking at.

The other figures are the day ring, the compass and the size map.