World Clock Comparison

In short A world clock is one instant shown on many clock faces. The times differ because the clocks differ, not because the moment does — and that distinction is the reason software should store an instant in UTC and treat local time purely as a display. Pick a moment and a set of zones above and every row is the same instant, with its local time, its local calendar date and the offset that produced it. Two things become obvious once it is laid out. The first is that the calendar date is not shared: across a wide enough spread of zones two or even three different dates are in progress at once, because the world's offsets span twenty-five hours from UTC−11:00 to UTC+14:00 rather than a neat twenty-four. A deadline of "the 15th" is therefore ambiguous by up to a day unless the zone is stated. The second is how little overlap there is between working days on opposite sides of the planet: 09:00 to 17:00 in London and the same in Sydney share almost no hours at all, which is why one side of a genuinely global team is always meeting outside its own day. The working-hours label here is that 09:00–17:00 convention, which is a reasonable default rather than a fact.