Decimal time was the law of the land for only a brief time. In 1793, during the French Revolution, decimal time was declared mandatory for public acts, starting on 1794 September 22, or 1 Vendémiaire, year III, in the new calendar. However, the law establishing the metric system on 18 Germinal, year III, (1795 April 7) suspended mandatory use just six months after it started. This did not entirely kill decimal time, though. It still continued to be used in some parts of France, such as Toulouse, for a number of years in official records, perhaps until Napoleon abolished the new calendar at the end of 1805, or maybe when ten-day weeks were dropped in 1802.
But even then, decimal time did not disappear completely. The great French astronomer and mathematician, Pierre-Simon Laplace, was enthusiastic about decimal time and had a decimal watch made. He also used decimal time in his work, Traité de Mécanique Céleste, in 1799. He represented the time of day as a decimal fraction, such as 0j,60566. He also added the fraction to the Gregorian calendar date, such as:
...l'instant du passage au périhélie, sept.29j,10239, temps moyen compté de minuit à Paris.
In English, that's "...the instant of perihelion passage, Sept. 29d.10239, mean time counted from midnight at Paris." This became common practice by astronomers, with French astronomers using mean time in Paris and British astronomers using Greenwich mean time. After the British astronomer John Herschel proposed Julian days, astronomers added decimal fractions to Julian days, creating Julian Dates, which are still in wide use in astronomy, along with variants such as Modified Julian Dates. Calendar dates with times given as decimals of a day in Universal Time have been used in astronomical circulars for the past century. Others, such as computer programmers, also use decimal fractions of a day to represent time.
So these modern applications of decimal time can be linked, through Laplace, to the decimal time established back in the 18th century during the French Revolution, and possibly inspired the stardates that are projected in future centuries.
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