World clock
The current time in as many places as you want on screen at once, each with its own date. The dates are the part worth watching: the inhabited world spans twenty-six hours of offsets rather than twenty-four, so two calendar dates are always in use somewhere, and for two hours out of every day there are three.
How to use the world clock
The offsets on this wall cover twenty-six hours, not twenty-four. They run from UTC−12 in the uninhabited central Pacific to UTC+14 in Kiribati, which moved its Line Islands to the western side of the date line in 1995 so that the whole country would share a working week. The effect is that Kiritimati begins a day while Honolulu, a couple of thousand kilometres away, is still finishing the one before. Because the span is wider than a day, there are always two calendar dates in use across the world, and from 10:00 to 11:59 UTC there are three at once.
Not every offset is a whole number of hours, which is why a converter built on integers is wrong for a couple of billion people. India runs on UTC+5:30 across a country wide enough to justify two zones. Nepal sits at +5:45, a quarter of an hour ahead of India rather than level with it. The Chatham Islands are at +12:45 and Newfoundland at −3:30, and the Australian state boundaries produce +8:45 at Eucla for a few dozen people.
Width is a political decision rather than an astronomical one, and the wall makes that visible. China spans five geographic zones and keeps a single clock across all of them, so in Kashgar in the far west the sun comes up around ten by the official time and the working day is simply shifted to suit. Russia runs eleven zones across its own landmass; France, counting its overseas territories, is spread across twelve, more than any other country.
What people use it for
- Checking whether it is a civil hour to phone someone abroad
- Keeping the offices you work with on screen through the day
- Knowing what date it already is on the far side of the date line
- Seeing when a market opens relative to where you are sitting
- Working out when family in another country will be awake
Questions
Yes, per zone and automatically, using the time zone database your browser ships with.
Two, most of the time. Between 10:00 and 11:59 UTC there are three, because the inhabited offsets span twenty-six hours.
Because countries chose them. India is at +5:30, Nepal at +5:45, the Chatham Islands at +12:45 and Newfoundland at −3:30.
France, at twelve once its overseas territories are counted. Russia has eleven across one contiguous landmass.
Policy. It covers five geographic zones on a single clock, which is why sunrise in Kashgar is around ten in the morning by official time.
No. Nothing is stored, so add them again next time.
Your own machine’s clock, offset per zone. A wall of clocks is only as right as the device it is drawn on.