When France Made the Day 10 Hours Long: Decimal Time

Revolutionary France, having decimalized weights, measures, and money, aimed its logic at the clock. A 1793 decree defined the day as 10 hours, each hour as 100 decimal minutes, each minute as 100 decimal seconds — midnight at 10, noon at 5, and every conversion a matter of moving a decimal point. Clockmakers produced handsome dual dials showing both systems; a decimal hour ran 144 old minutes, a decimal minute 86.4 old seconds.

The republic paired it with a decimal calendar: twelve 30-day months named for weather and harvest — Thermidor, Brumaire, Floréal — each divided into three 10-day “décades,” with five festival days closing the year. Mathematically elegant, humanly brutal: one rest day in ten instead of one in seven proved wildly unpopular, and every existing clock, watch, timetable, and habit stood in the way of the new hours.

Decimal time lasted barely two years as an official requirement before being suspended in 1795; the calendar limped on until Napoleon abolished it in 1806. The failure taught a durable lesson about standards: the meter replaced a chaos of regional units and won, but the 24-hour day and 7-day week were already universal — and a standard everyone shares is nearly impossible to dethrone, however inelegant its arithmetic.

Astronomers quietly kept the useful part. The Julian Date system counts days and decimal fractions of a day, so an observation at 18:00 UTC is simply.75 — decimal time in professional disguise, running inside every ephemeris and, indeed, inside the solar calculations behind sunrise tables.

The dream resurfaces periodically. In 1998 Swatch launched Internet Time, dividing the day into 1000 “.beats” (@500 as midday in its own meridian at Biel), marketed as one global time for the online age; it decorated some watches and one search engine’s early pages before fading into trivia.

So the day remains 86,400 stubborn seconds — sexagesimal Babylon defeating decimal Paris — and the clocks at itsdately.com count it the ancient way, as, in all likelihood, clocks always will.

The scheme’s rational core wasn’t wrong — computers happily count decimal fractions of days, and scientists never stopped wishing arithmetic on the clock — it was simply late. Sundials, canonical hours, bells, watches, and human habit had compounded for millennia; the revolution could rename the months by decree but could not re-teach every body’s sense of when lunch happens.

Collectors now prize the era’s relics: dual-dial decimal watches and clocks survive in museums, their faces split between 10 and 12, frozen mid-argument. They make a fitting monument — beautiful, precise, and stopped — to the one metric battle Paris lost.

Historians of measurement treat the episode as the exception that maps the rule: units succeed when they replace confusion, and fail when they replace consensus. The kilogram unified a thousand quarrelling local pounds; the decimal hour attacked the one measure on which every village, church, and sailor already agreed. Reformers since have aimed lower and won — standard zones, ISO date formats, the 24-hour timetable — each a decimal-spirited tidying that left the ancient 60s untouched at the core.

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Related on itsdately: Where the Month Names Come From: Gods, Emperors, and a Counting Error · Why Days Are Named After Planets and Gods · Why Does a Week Have Seven Days?

Further reading: Encyclopaedia Britannica — Calendar.

Related read: Why Does February Have Only 28 Days?

Related read: What Is a Fortnight? The 14-Day Word That Refuses to Fade.