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The Gregorian Calendar, 1582
Among the most dramatic shake-ups in western timekeeping was the changeover from the Julian to Gregorian calendar in the 16th century. In much of western Europe, this saw 10 days "disappear" in a readjustment that brought the date back in step with the seasons.
The Julian calendar had been introduced by Julius Caesar and his reforms had remained in place for more than 1,500 years. But the calculations on which it was based, that the year lasted 365 and a quarter days, was out by about 11 minutes. Over a few decades, this wasn't important, but as the centuries passed, the discrepancy grew.
By the 16th century, this difference between the calendar and the seasons had grown to 10 days. So in 1582 Pope Gregory XIII cut the year short and created a more complex system of leap years which we still use today.
This innovation was at first not accepted in Britain or its colonies, which meant there was a 10-day difference between London and Paris. This was not rectified until 1752 when Britain adopted the Gregorian calendar, which by then meant cutting 11 days from the year.
Under the Calendar Act, the day following September 2 was September 14 which caused widespread protest about lives being cut short. People said "give us back our 11 days," although steps had been taken to ensure they did not lose wages or miss birthdays. This reform also saw January 1 become new year's day, rather than March 25.
The prospect of losing 11 days was also not welcomed by financiers. And so that they wouldn't lose out, the start of the financial year was pushed back 11 days to the current April 6.
The arrival of the railways in the first half of the 19th century was one of the great driving forces towards a standardisation of time keeping.
Even if the time could be measured accurately in one place, it need not be the same as somewhere else in the country because noon, identifiable by a sundial, varied depending on how far east or west you were. Noon in London came five minutes after Norwich and 16 minutes before Plymouth and it was not unusual for watches and clocks to have two minute hands, showing both the local time and London time.
But when railways arrived, timetables meant there had to be a single time for departures and arrivals, which became known as "railway time" and was used regardless of local practice.
Assisting this push towards a common standard was the distribution by telegraph in 1852 of an official time from the Royal Observatory in Greenwich which gave clocks around the country a reference point.
But the adoption of Greenwich Mean Time was not legally enforced until 1880, and until that time there were many local variations.
Travellers at the railway station in Gloucester had to contend with three clocks showing different time zones in Bristol, Birmingham and London.
And a court case in Dorchester was lost when one party failed to turn up at court on time. When they did arrive, they claimed they were on time, but it was local rather than London time. There were also arguments over when polling booths should close during general elections.
For businessmen in London a woman had the job of setting an accurate portable timepiece, made by John Arnold, against the Greenwich clock and then taking it to the City. This official version of time became known as "Arnold" and was carried to clients in the City in a handbag.
This "mean time" was based on an average of the length of a day; only a few days are exactly 24 hours, but for time-keeping a standardised day was essential.
After standardising time within one country, the next stage was to set international time so that there could be a single system for travellers and traders around the world.
In 1884 an International Meridian Conference was held in Washington, which agreed to divide the globe into 24 segments of time, to be known as "time zones" - with 15 degrees of longitude representing a band of one hour.
Once across this imaginary boundary, clocks moved forward or backwards by an hour.
Britain was then the global superpower and with the support of the United States the observatory at Greenwich was agreed to be the prime meridian.
The conference also agreed that all countries would accept a universal standard for the length of a day.
Greenwich Mean Time has since been superseded by Co-ordinated Universal Time (UTC - the acronym decided on) as the world standard for time-keeping.
This definitive measure is set by the International Bureau of Weights and Measures in Paris, taking an average of more than 200 atomic clocks and then adding any necessary "leap seconds" to keep this official time in step with the rotation of the earth.
While the atomic clocks might be almost perfectly constant, the earth isn't. And when the rotation of the planet shows a slight fluctuation, the setting of UTC has to be adjusted. For instance, a second had to be added to the end of 1998.
And there are international standards about the duration of the units of time. The second would once have been measured only as a fraction of an hour, which was itself a fraction of the day, which was defined by the observations of the night and day.
And in case anyone asks you the time, you can quote the International System of Units and tell them a second is "the period equal to 9,192,631,770 cycles of the radiation which corresponds to the transition between two energy levels of the ground state of the cesium-133 atom."
French Revolution, 1793 to 1806
The Republican calendar, introduced in France in 1793 in the wake of the revolution, swept away religious and historical associations and imposed science and reason.
1792 was designated Year 1 and months were re-named after nature: Vendemiaire, Brumaire, Frimaire, Nivose, Pluviose, Ventose, Germinal, Floreal, Prairial, Messidor, Thermidor, Fructidor. The week was abolished and replaced by three 10-day periods, with the final day considered a day of rest. The days were also re-named in numerical order, from "Primidi" to "Decadi".
New Year celebrations were to be held on the autumn equinox (September 22) and to make the length of the calendar match the natural year, a number of days were tagged on after the final month, with names such as "Reason Day" and "Labour Day".
With nine days between rest days, the calendar was not popular and was abolished by Napoleon at the beginning of 1806.