EOY

How time zones work

The Earth turns 360° in 24 hours, so every 15° of longitude is an hour of clock time and every degree is four minutes. Time zones are that arithmetic, smoothed for convenience and bent by politics.

Longitude, not latitude

Noon is when the Sun is highest in the sky, and the Sun is highest at different moments for places east and west of each other. Two towns on the same north–south line — the same longitude — see noon at the same instant, however far apart they are; two towns on the same latitude but 15° apart in longitude see it an hour apart. Latitude changes how long the day is and how high the Sun climbs, not what time it is. That is why the zone map is sliced into vertical strips, not horizontal bands.

From local time to standard time

Before standard time, every town kept local mean solar time: noon by the Sun overhead, so clocks a few dozen kilometres apart disagreed by a few minutes. Railways made that unworkable. Great Britain's railways adopted a single railway time in the 1840s, and on 18 November 1883 the North American railroads switched to standard zones one hour apart. A year later the International Meridian Conference in Washington chose Greenwich as the prime meridian — the zero line from which everything else is counted — though, contrary to popular belief, it did not create the time zones; countries adopted those over the following decades.

UTC and offsets

Today every zone is defined as an offset from Coordinated Universal Time. Offsets run from UTC−12 to UTC+14 — 26 hours, not 24, because the date line bends to let the Line Islands and Samoa sit a full day ahead of the Pacific islands east of them. Most offsets are whole hours; India (UTC+5:30), Nepal (UTC+5:45) and a handful of others use half or quarter hours — see the odd offsets.

Why the map is bent

If zones followed longitude exactly, each would be a 15° strip. Real zones follow borders, provinces and trade. China spans almost five geographical zones of longitude (73°E to 135°E) and keeps a single national time, UTC+8, so in the far west of Xinjiang the Sun can rise after ten in the morning by the clock. The strips are a starting point; the actual lines are drawn by governments.

What the countdown does with this

The board treats the zone as an input: pick one from the list and it computes the exact instant of midnight on 1 January there, from the zone's own offset rules, including any daylight saving change between now and then. That is why New Year arrives 26 hours apart across the world and why the board can show each arrival in turn.

Sources

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