If you’ve traveled internationally or (if you’re from the US) taken any chemistry or physics classes, you’ve probably encountered the challenge of converting between unit systems. Even if you’ve worked with them before, different temperatures, distances, and masses can all be daunting or at least annoying.

At least there’s one thing that everyone agrees on in daily life, and that’s time. Seconds are seconds, and usually minutes are minutes and hours are hours. This seems a little strange, actually, when you think about it. Why is time measured the same way in the metric and English systems?

To answer that, we have to go back to the introduction of the metric system, a time of upheaval, idealism, and decapitation. That’s right, I’m talking about the French Revolution! (Yes, this is a very late Bastille Day post.)

Today we all know meters and grams, and even liters, which aren’t actually an official metric unit. However, it’s less well known that the regimented, decimal-based system was originally supposed to include a comprehensive reform of time as well.

To understand why, remember that there was kind of a lot going on in France between 1789 and 1799, including the academic and scientific community trying to justify its existence as a totally-not-elitist institution of the people. In their zeal to prove their worth to the Republic, scientists embarked on a number of scientific and social initiatives, combining progress with republican spirit. One of these projects was the push to end the jumble of local units in use at the time (as many as a quarter million, according to some estimates!) and finally put the metric system to practical use.

Forms of the metric system had been proposed before, and its adoption in France was actually the result of a pre-existing international effort with Britain and the new United States. When other nations started getting politely concerned about the overthrow of the French monarchy and the resulting chaos, the Anglophones backed out and France was left to pursue the new metric system alone.

The change to decimal time was rolled out ahead of the meter and gram, the original mainstays of the current metric system (or SI for Système International—see, French!). There were three contenders for defining the meter, but eventually the definition as a fraction of the length of the meridian or longitude line running through Paris won out, maybe because it had more patriotic spirit than the other options. The gram was defined in terms of the meter, as the mass of water that fit in one cubic centimeter.

Approving the definition of the meter was only the first step, followed by years of studies and expeditions to get the most accurate possible measurement of the meridian length. By contrast, the length of a day was already pretty accurately known. Thus, decimal time got a head start with its introduction in a November 1793 decree and passed into official use in September of 1794.

Since nobody was up for trying to change the length of the day (probably a wise choice), that left the units around it, both larger and smaller. With a more rational system, there would be no more of those silly 60-second minutes, 60-minute hours, or 24-hour days. Instead, each day was divided into 10 hours, each hour into 100 minutes, and each minute into 100 seconds.

This left the good French people with hours that were more than twice as long as they had always been, but the minutes and seconds weren’t too bad. The decimal minute was equivalent to 1.44 silly old minutes, and the decimal second was 0.864 obsolete, non-Republican seconds.

There were also changes on the larger end of the scale, though obviously limited. After all, the only units we regularly use that are larger than days are weeks and months, and then we run into years, which are again pretty non-negotiable. The new Republican Calendar had ten-day weeks grouped into three-week months, with an extra five or six days to dispose of at the end of the year.

Unlike the smaller-scale decimal time, the months had all-new names based on natural features of the seasons, like Brumaire (from French brume, “mist”) and Thermidor (from Greek thermon, “summer heat”). These represented a determined break from the traditions that had governed the calendar in Romance languages since the Roman Empire. When it came to republican innovation, the French didn’t mess around.

Scrapping your time units and starting over sounds pretty daring to me, if only because people use them all the time. Not everyone has a scale or ruler handy, or even a good intuitive idea of what different masses, volumes, or lengths look like. Pretty much everyone had access to a clock in some form by the end of the eighteenth century, though, even if it was something more approximate like a water clock or hourglass. Hours would’ve been used for every aspect of life in France, and the shift from twenty-four of them to ten was a big one. That’s just the personal inconvenience—let’s not even get into the cost of making new clocks for public and private use across the nation.

In the end, the new timekeeping was apparently too much to cope with, on top of all the other changes sweeping the country. Decimal time was retired from mandatory public use in April 1795, less than two years after its introduction. The same law that dropped decimal time as the official standard also introduced the units for the metric system, which took several more years to be measured and calculated to everyone’s satisfaction.

The Republican Calendar lasted about ten years longer, until 1806. The rest of the metric system spread through Europe thanks to France’s military, commercial, and scientific influence. Today it’s used by most of the world and pretty much all scientists. (I’m sure some of us work in other units, but for goodness sake, don’t forget to convert!)

Even after it was no longer required, decimal time didn’t die out entirely. Cities like Toulouse and Marseille continued to use it for a few years, and there was even an ill-fated proposal to reinstate it several years after its retirement. It lives on in science to this day as the fractional days often used in astronomy, thanks to Pierre-Simon Laplace, who famously wore a decimal watch and used decimal time in his work.

In retrospect, I wonder whether part of the reason for decimal time’s failure was the attempt to use the same old units to suddenly mean completely different things. It’s hard enough to introduce a totally new unit, but it’s another thing entirely to announce that from now on each hour will be 2.4 times as long as it used to be. Personally, I doubt I’d be a fan.

Whatever the practical and social pressures that led to the demise of decimal time, it was certainly an ambitious idea. Like many of history’s most interesting projects, it was an attempt to combine science with the spirit of the times, and the popularity of the metric system suggests that it wasn’t totally crazy. It’s interesting to look at the false starts of history and wonder what our descendants will think of some of our scientific fads.

[The Revolution wasn’t the last time existing units have been tweaked in the name of scientific accuracy—since those days, units like the meter and second have been formally redefined multiple times and are currently measured in terms of the speed of light and atomic energy level transitions, respectively. Also, check out this slightly sensationalized but fun article on the change in the definition of the kilogram!

Read more about the history of the metric system, and check out this article for its picture of a dual clock face with both 10 and 24 hours!]