Before standard time
For most of human history, noon was whenever the sun was highest. Every town therefore kept slightly different time. Cardiff ran roughly thirteen minutes behind London, and Bristol about ten. Nobody was inconvenienced, because nobody could travel fast enough for the difference to matter.
The railways forced the issue
That changed with the railway. A train leaving London at ten and arriving in Bristol at noon local time created timetables that were not merely confusing but dangerous, since trains sharing a single track relied on synchronised signalling. The Great Western Railway adopted London time across its network in 1840, and by 1855 most public clocks in Britain showed Greenwich Mean Time. Legal recognition took longer, GMT did not become the statutory basis of British time until the Statutes (Definition of Time) Act of 1880.
1884: the world agrees on a meridian
In October 1884, delegates from twenty-five nations met in Washington for the International Meridian Conference. They adopted Greenwich as the Prime Meridian, fixed the universal day as beginning at mean midnight there, and endorsed a system of hourly zones. The choice was pragmatic: most of the world’s shipping charts already used Greenwich. France abstained and continued to define its time against Paris until 1911, initially by the diplomatic fiction of describing it as “Paris Mean Time retarded by nine minutes and twenty-one seconds”.
Daylight saving
The idea of shifting clocks seasonally was proposed by the New Zealand entomologist George Hudson in 1895 and independently by the British builder William Willett in 1907, who disliked finding the morning light wasted while riding before breakfast. Willett died in 1915 without seeing it adopted. Germany introduced it first in 1916 to conserve coal during the First World War, and Britain followed within weeks.
Britain has experimented since. British Double Summer Time ran two hours ahead of GMT during the Second World War. Between 1968 and 1971 the country remained on GMT+1 year-round; the trial ended after sustained objection to dark winter mornings in Scotland and north Wales, where sunrise would not have arrived until after 10:00am.
From astronomy to atoms
Greenwich Mean Time was originally derived from observing the stars, but the earth is not a reliable clock, its rotation is gradually slowing and fluctuates unpredictably. In 1967 the second was redefined in terms of caesium atoms, and Coordinated Universal Time took over as the world’s reference. Leap seconds were introduced to keep UTC aligned with the turning earth; in 2022 the international community voted to retire them by 2035, having concluded that the disruption they cause to computer systems outweighs the astronomical tidiness.
Today the practical record of all this lives in the IANA time zone database, maintained collaboratively and updated several times a year as governments change their minds. It is the reason a browser can tell you not just the time in Cardiff now, but what it was on any date in the past century.