For most of human history, noon was a local fact. It was the moment the sun stood highest over your particular town, and every town kept its own, so the clock in one city ran minutes off the clock in the next. That was tolerable when a journey between them took days. It became chaos once the railways could cross those minutes in an hour, leaving timetables that no traveller could trust, because every station along the line told a slightly different time.

On 18 November 1883, the railroads of the United States and Canada simply agreed to stop. They divided the continent into four broad zones and set every clock in each zone to a single shared time, replacing dozens of local suns with a handful of official ones. For towns that sat east of their new zone's meridian, the shared time ran behind their old solar time, and so, at the changeover, their clocks were wound backwards in the middle of the day. Noon arrived, and then a little later, on the new reckoning, noon arrived again. People lived through the same midday twice, and the date earned its nickname: the day of two noons.

The law took its time catching up to the railways. It was not until 1918, with the Standard Time Act, that the United States gave this arrangement the force of legislation, formalising what the trains had already imposed. Britain had walked a similar road decades earlier, its railways adopting London's Greenwich time in the 1840s to keep their own timetables honest. In every case the pattern was the same: the sun kept perfect local time for everyone, and it was the machine, the train that could outrun the difference, that finally made us give the sun up and agree on a clock we could share.