Why time zones exist — and the day noon happened twice.

Before standard time, clocks were set by the sun: noon was the moment the sun stood highest over your town. Neighbouring cities disagreed by minutes as a matter of simple geography — and nobody minded. The disagreement only became a problem when people started moving faster than the sun's own shadow.

Trains made minutes matter across distances. British railways began running on London's Greenwich time in the 1840s — “railway time” — often shown alongside the local time, so a station clock disagreeing with the town clock outside became the visible seam of the old world.

On November 18, 1883, the railroads across the United States and Canada switched to Standard Railway Time — four broad zones in place of dozens of local suns. In cities east of their new zone's meridian, clocks were set back mid-day, and noon happened twice. US law only caught up in 1918, with the Standard Time Act.

The Canadian railway engineer Sandford Fleming became the great advocate of a worldwide system of hour-wide zones — the story goes that missing a train in 1876 over an ambiguous timetable set him off. At the International Meridian Conference in Washington in 1884, delegates chose Greenwich as the prime meridian, and the world's clocks gained a common zero.

All of that history now fits in a handful of characters. Z stands for UTC, Greenwich's descendant; ±hh:mm names the zones the railroads carved out. The tokens on this site are the 1884 conference, compressed.

The extended offset — -07:00, UTC offset extended — is what a JSON timestamp or an HTML datetime attribute wants; its colon-less twin is the -0700 basic form. And the short zone name (MST) is the ambiguous cousin the offset was invented to replace. See how every language spells them in the token matrix.