Decimal time

The seventeen months it was the law

Revolutionary France metricated everything it could reach — length, weight, the calendar — and the day was an obvious target. A day divided by ten, then by a hundred, then by a hundred again gives 100,000 decimal seconds, every unit a round fraction of the whole. Decimal time became official on 24 November 1793 and was suspended on 7 April 1795.

It failed for the reason metric time keeps failing: the day is the only unit anybody was arguing about, and replacing it means replacing every clock in the country at once. Watchmakers hedged by building dials with both scales, decimal on the inside ring and the old twelve hours on the outside; a few of those survive in museums and they are the most legible argument against the whole project.

Doing it by hand is easy in one direction and annoying in the other. Ordinary to decimal: seconds since midnight, divided by 0.864. Decimal to ordinary: multiply by 0.864 and turn the result back into hours, minutes and seconds. Midday, 43,200 seconds in, lands on exactly 5:00.

The idea keeps coming back under the name metric time, and it is worth knowing that the metric system itself never adopted it: SI kept the second and built everything on top of that instead, which is why a metre-kilogram-second world still tells the time in sixties.