During the French Revolution, the French invented and put into use a new Revolutionary calendar. The Revolutionary calendar was established in October 1793, but Year I was made effective on September 22, 1792 (both the autumnal equinox and the anniversary of some event of the Revolution; I'm not sure exactly which). The Revolutionary calendar had 12 months of 30 days, plus 5 or 6 leap days (with a rule for leap years).

The autumn months were given the names Vendémiaire, Brumaire, and Frimaire; the winter months were called Nivôse, Pluviôse, and Ventôse; the spring months were called Germincal, Floréal, and Prairial; and the summer months were called Messidor, Thermidor, and Fructidor. The extra days were called Sans Culottides.

Each month consisted of three weeks of ten days. The days were named after various crops and flowers. Instead of getting one day off every seven to pray in a church, the peasants were miffed that they got off only one day in ten to attend a "temple of reason," which is what the churches were rechristened. The years were numbered starting with the revolution, Year 1. The French also established a new clock, in which the day was divided in ten hours of a hundred minutes of a hundred seconds. (Thus, exactly 100,000 seconds per day.) The French Revolutionary calendar was in use from 1793 through 1805. Napoleon re-instituted the Gregorian calendar on December 31, 1805, as well as abolishing the "temples of reason," and the new clock. However, the metric system, established at the same time, remained, and has caught on through most of the world (with the notable exception of the United States).